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.
In more detail
Read to the bottom of the sheet and you meet a sentence that has only just started. There is nothing under it, so you turn over to find out what it was going to say. The paragraph has been opened on one page and told on another, and the one line left behind does no work at all.
There is a second stranded line, the mirror of this one: a paragraph that finishes at the top of the next page and leaves a single line there. That one is called a widow line. The two names get swapped from one workshop to the next, so what tells them apart is the position: the orphan is alone at the bottom, the widow is alone at the top.
The cure is the same in both cases and it has never changed: rather than cutting the paragraph where the page happens to end, the cut is moved a line or two back up, so that a decent handful stays on each side. When even that is not enough, the whole paragraph goes over to the next page in one piece.
What it does on a real document
A document laid out by a program has no eye to catch it. The text is poured into a frame, the frame fills up, and whatever is left goes into the frame on the sheet after — one line as readily as twenty. So the count is stated instead: how many lines of a paragraph must stay at the foot of a page for the break to be allowed there.
Nothing is asked for on an ordinary invoice of a single page, because nothing is cut. It starts to matter the moment a document runs long: terms and conditions behind an order form, a contract of twenty pages, a book, a report of general conditions attached to a policy. Those are exactly the documents somebody signs, and a page that opens with a fragment reads as carelessly made.
When the document asks for nothing, this engine keeps 2 lines on each side of the break, which is the count a printer would have used without being told. A document that wants more says so, paragraph block by paragraph block, and a table that is allowed to cut a row in the middle of its text says it for a cell of that row.
Where it comes from, and why
It comes from the composing room, where a page was built by hand out of metal letters and a single line at the wrong end of it was one of the first faults an apprentice was taught to look for. The trade even kept a saying to tell the two apart: an orphan has no past, a widow has no future.
The reason was never decoration. A book is read across a pair of facing pages, and a line stranded at a corner of that pair breaks the block of grey the eye follows. Whoever pays for the printing sees it at a glance, which is why it has been a rule for centuries rather than a preference.