means a small mistake in typed or printed text, usually a misspelling or wrong keystroke rather than an error of knowledge.

from A 20th-century clipping of "typographical error," the trade phrase printers used for mistakes made at the type-case or keyboard rather than by the author. "Typographical" itself reaches back through Latin to the Greek typos, "an impression or stamp" — the mark left when something is struck. So a typo is, quite literally, a wrongly struck impression, shortened to a snappy little word the moment people typed fast enough to make plenty of them.