in Latin, the form of com- "together, with" in compounds with stems beginning in vowels, h-, and gn-; see com-. Taken in English from 17c. as a living prefix meaning "together, mutually, in common," and used promiscuously with native words (co-worker) and Latin-derived words not beginning with vowels (codependent), including some already having it (co-conspirator).
1510s, "one who steers a ship," especially one who has charge of the helm when the ship is passing in or out of harbor, from French pillote (16c.), from Italian piloto, supposed to be an alteration of Old Italian pedoto, which usually is said to be from Medieval Greek *pedotes "rudder, helmsman," from Greek pedon "steering oar," related to pous (genitive podos) "foot," from PIE root *ped- "foot." The change of -d- to -l- in Latin-derived languages ("Sabine -l-") parallels that in odor/olfactory; see lachrymose.
The transferred or figurative sense "a guide, a director of the course of others" is by 1590s. The literal sense was extended by 1848 to "one who controls a balloon," and by 1907 to "one who flies an airplane."
As an adjective, 1788 as "pertaining to a pilot;" from 1928 as "serving as a prototype," thus the noun pilot meaning "pilot episode" (etc.), attested from 1962. A pilot light (by 1890) is a very small light kept burning beside a large burner to automatically light the main burner when the flow is turned on.