colour space
A colour space is the rule that turns a set of numbers into a colour somebody can see.
In more detail
A colour written in a file is only numbers. On its own, a set of three says nothing: it is one red on a telephone, another on a laptop, and a third once ink is laid on paper. The space is the sentence that goes with the numbers and says how to read them.
Some spaces describe a device — the lights of a screen, the four inks of a press — and give the same numbers a slightly different answer on each machine. Others describe the colour itself, measured, and give the same answer everywhere. A document that matters chooses the second kind, or pins the first one down with a profile.
What it does on a real document
A page may name a colour in the three lights of a screen, in the four inks of a press, in a single grey, in a named ink of its own, or in a measured space carried inside the file. The file holds whichever spaces it uses, so it reads the same decades from now as it does today.
A request states a space once, under a name, and every colour of the document that wants it refers to that name. Nothing needs restating on each mark.
Where it comes from, and why
Colour had to be written down long before there were screens: the measurements the modern spaces rest on were taken in the nineteen thirties, from people asked to match one light against another.
Everything since has been a way of tying a machine to those measurements, so that a colour ordered in one place comes out the same in another.