thin space

If it be only a Single Letter or two that drops, he thruſts the end of his Bodkin between every Letter of that Word, till he comes to a Space: and then perhaps by forcing thoſe Letters closer, he may have room to put in another Space or a Thin Space; which if he cannot do, and he finds the Space ſtand Looſe in the Form; he with the Point of his Bodkin picks the Space up and bows it a little; which bowing makes the Letters on each ſide of the Space keep their parallel diſtance; for by its Spring it thruſts the Letters that were cloſed with the end of the Bodkin to their adjunct Letters, that needed no cloſing.

All you may see on the keyboard is a space bar, but typographers use several invisible characters: the word space, fixed spaces of various sizes (em space, en space, thin space, figure space, etc) and a hard space or no-break space.