token
1. <grammar> A basic, grammatically indivisible unit of a language such as a keyword, operator or identifier. Compare: lexeme.
2. <convention> (Or "pumpkin") An abstract concept passed between cooperating agents to ensure synchronised access to a shared resource. Such a token is never duplicated or destroyed (unless the resource is) and whoever has the token has exclusive access to the resource it controls. See for example token ring.
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.