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Old English tacen "sign, symbol, evidence" (related to tæcan "show, explain, teach"), from Proto-Germanic *taiknan (cf. Old Saxon tekan, Old Norse teikn "zodiac sign, omen, token," Old Frisian, Middle Dutch teken, Dutch teken, Old High German zeihhan, German zeichen, Gothic taikn "sign, token"), from PIE root *deik- "to show" (see teach).
Meaning "coin-like piece of stamped metal" is first recorded 1590s. Original sense of "evidence" is retained in by the same token (mid-15c.), originally "introducing a corroborating evidence."
"nominal," 1915, from token (n.). In integration sense, first recorded 1960.
If several programmers are working on a program, one programmer will "have the token" at any time, meaning that only he can change the program whereas others can only read it. If someone else wants to modify it he must first obtain the token.