Every word used on this site, explained

Every word this site has to use, and what it means.

PDF library: A PDF library is a piece of software your own programs use to write PDF documents.
reading software: Reading software is the program that opens a PDF and puts it on a screen.
byte: A byte is the unit a file's weight is counted in, the way a parcel is counted in grams.
compression: Compression is writing the same document in fewer bytes, with nothing taken out of it.
object streams: Object streams are where a document's bookkeeping is packed away, so that it takes a fraction of the room it would take in plain view.
cross-reference table: A cross-reference table is the map at the end of a PDF, saying where in the file every numbered part of the document begins.
linearised PDF: A linearised PDF is written so that its first page can be shown before the whole file has been received.
typographic point: A typographic point is the unit printing measures with, worth a little over a third of a millimetre.
standard: A standard is a rule argued out in committee, published under a number, and identical for everybody who claims it.
PDF/A: PDF/A is a PDF that carries everything it needs inside itself, so that opening it decades from now does not depend on anything else still being around.
PDF/UA: PDF/UA is a PDF whose text carries its own structure, so that software reading the page aloud says the right things in the right order.
Ghostscript: Ghostscript is a free program that opens a PDF file and says what is written inside it, without deciding what it means, and it can create one too.
veraPDF: veraPDF is a free program that opens a document and says whether it really follows the rule it claims to follow.
Factur-X: Factur-X is an invoice that is a printed page for a person and a data file for a machine, in one single document.
attachment: An attachment is a whole file tucked inside a PDF and carried along with it, the way a paper clip holds a sheet to a folder.
the identity card of a file: XMP is a metadata packet written inside a file that says what the file is: its title, who made it, when, and which rules it follows.
colour profile: A colour profile is a small file that says what a colour actually looks like, so that the same numbers give the same colour on another machine.
output intent: The output intent is the colour profile written into a document to say which press, or which screen, its colours were chosen for.
print standard: A print standard is the published rule a file has to satisfy for a printing works to run it exactly as it arrives.
PDF/X: PDF/X is a PDF a printing works can put straight on the press, without ringing anybody to ask what was meant.
CMYK: CMYK is the four inks a printing press lays down: cyan, magenta, yellow and black.
RGB: RGB is the three lights a screen mixes a colour from: red, green and blue.
spot colour: A spot colour is an ink the printing works keeps ready mixed and prints from a plate of its own.
colour separation: Colour separation is splitting a page into one drawing per ink, because a press lays its inks down one at a time.
printing plate: A printing plate is the metal sheet a press prints one ink from.
blade: The blade is the knife that cuts a printed sheet down to the finished page.
bleed: Bleed is the colour that runs past the line the blade will cut.
trim marks: Trim marks are the short lines printed outside the finished page to show where the blade goes.
alignment crosses: Alignment crosses are the small crosses printed outside the finished page so that a printing works can lay its plates exactly over one another.
guillotine: A guillotine is the machine a printing works cuts its sheets on: one long blade, brought down on a whole stack at once.
trapping: Trapping is letting two touching colours run a hair into one another, so that no white line opens along the join.
overprint: Overprinting is letting an ink lie over the ink already on the sheet, instead of wiping it off.
halftone screen: A halftone screen is the grid of tiny dots a press builds a shade out of.
rendering intent: A rendering intent is the rule that says what becomes of a colour the destination cannot hold.
font: A font is the drawing of every letter, digit and mark a document writes.
lightened font: A subset is the part of a font a document actually draws, copied into the file in place of the whole font.
glyph: A glyph is one drawn shape of a font. The letter A is the letter; the particular drawing of it that lands on the page is a glyph.
justified text: Justified text is text whose lines all stop on the same right edge.
soft hyphen: A soft hyphen marks a place inside a word where it may be broken, and is drawn only if the break falls there.
no-break space: A no-break space looks like any other space, but the line is never allowed to end on it.
gradient: A gradient is a colour that changes across the space it fills, with no step anywhere between one end and the other.
transparency: Transparency is paint that lets what is underneath it come back through.
opacity: Opacity is how much of what is already on the page a mark hides, said as a number from zero to one.
flattening: Flattening is crushing a page built in several layers into one flat drawing, so that nothing shows through anything any more.
tagged PDF: A tagged PDF carries the structure of its own text beside the drawing: this is a heading, this is a table, this cell belongs to that column.
structure tree: The structure tree is the list, held inside a tagged PDF, that says what each part of the page is and what it belongs to.
reading order: The reading order is the order a document is meant to be read in, which is not always the order its marks were drawn in.
tab order: The tab order is the order the tab key walks through what a page can be visited on: its fields, its links, everything a reader can land on.
text alternative: A text alternative is a sentence written into the document to stand in for a picture, read out to somebody who cannot see it.
screen reader: A screen reader is software that says a document aloud for somebody who cannot see it.
barcode: A barcode is a set of bars and gaps standing for a number a scanner reads back in one pass.
2D barcode: A 2D barcode is read in two directions at once, so it holds far more than the striped kind.
QR code: A QR code is the square of black and white dots you point a telephone at: on a poster, on a restaurant table, at the foot of a bill.
DataMatrix: DataMatrix is a tiny square code, sometimes smaller than a grain of rice, for objects with no room for anything bigger.
Aztec: Aztec is the code with a little target in the middle, the one on a train ticket or on a boarding pass held up on a telephone screen.
PDF417: PDF417 is a wide, flat code made of stacked bars, the one on the back of a driving licence or on an airline boarding pass.
electronic seal: An electronic seal is the mark inside a document that proves who issued it, and that not one byte has moved since it was sealed.
signature: A signature on a PDF is a seal inside the document that proves who approved it, and that not one byte has changed since.
bookmark: Bookmarks are the list of headings reading software shows down the side of a document.
bead: A bead is one piece of page belonging to an article, threaded on it in the order it is read.
binding: A binding is the layer that lets a program in one language use code written in another.
service key: A service key is a long secret string your program sends with each call, so that the service knows the call is yours. Most software calls it an API key.
request: A request is one call to the service: you send what the document should say, and you get the document back.
template: A template is a PDF made once and reused as the background of new pages: your letterhead, a plain sheet, the terms you always attach at the end.
API: An API is the set of calls one program offers another: an address to send to, a shape to send in, and an answer that comes back as data.
HTTP: HTTP is the language a browser or a program uses to ask a machine on the network for something, and the shape the answer comes back in.
JSON: JSON is a way of writing structured information as plain text: named values, lists, numbers and words, nested as deep as needed.
server: A server is a program that waits for calls and answers them, running on a machine with nobody sitting at it.
reverse proxy: A reverse proxy is the program a call reaches first: it answers at the public address, checks the call, and passes it on to the machine that creates the PDF.
status code: A status code is the three-digit number an answer opens with, saying how the call went before anything else is read.
multipart form: A multipart form is one call carrying several named things at once, text and files alike, each in a part of its own.
base64: base64 is a way of writing any file using only ordinary letters, digits and two punctuation marks, so that it can travel inside text.
layer: A layer is a named group of things drawn on a page that whoever opens the file can switch on and off.
annotation: An annotation is something laid over a page rather than drawn into it: a clickable rectangle, a box to type in, a button.
form field: A form field is a box on a PDF page somebody can fill in, tick or choose from, and whose answer the file keeps.
baseline: The baseline is the invisible line letters sit on, the one ruled across a page of exercise paper.
Bézier curve: A Bézier curve is a curve described by its two ends and two more points that pull it out of line.
Python wheel: A Python wheel is the file a piece of Python software is delivered in, already built and ready to be put in place.
Rust crate: A Rust crate is the unit a piece of Rust software is delivered and named in: one library, under one name, at one version.
watermark: A watermark is wording printed across the page itself, usually to say what the document is: a draft, a copy, or a file made on a free account.
colour space: A colour space is the rule that turns a set of numbers into a colour somebody can see.
graphics state: A graphics state is the bundle of settings that is in force while a shape is being drawn.
TrueType: TrueType is one of the two shapes a font file comes in, and the one most fonts are delivered as.
OpenType: OpenType is the wider shape a font file comes in: the letters, and the typographic refinements that go with them.
Type 1 font: A Type 1 font is the older of the two shapes a font file comes in.
page label: A page label is the number reading software shows for a page, as opposed to where that page physically sits in the file.
running head: A running head is the line repeated at the top of every page to say where in the document the reader is.
blend mode: A blend mode is the rule for the colour that comes out where one mark is laid over another.
kerning: Kerning is the closing up of the gap between two particular letters that would otherwise look badly spaced.
ligature: A ligature is two letters drawn as a single shape, because set separately they would run into each other.
orphan line: An orphan line is the opening line of a paragraph left on its own at the foot of a page, with the rest of the paragraph on the next one.
widow line: A widow line is the closing line of a paragraph carried on its own to the top of a page, with everything above it left on the page before.