In Common Lisp and other Lisp-2s, short for function. When applied to a symbol, it returns the function binding of that symbol. When applied to a lambda expression, it returns a lexical closure formed from that lambda. Until the lambda macro was introduced, it was also necessary to use #' before lambda expressions when passing them as arguments to other functions.

In Emacs Lisp, #' is not necessary. Higher-order functions search both the variable and function bindings of their arguments. Likewise, a lambda expression evaluates to the lambda it expresses (and there are no true lexical closures: they must be simulated with the lexical-let macro). However, #' allows its argument to be byte-compiled, whereas quote or a plain lambda would not.

Examples:

(apply #'+ '(1 2 3))
(mapcar #'(lambda (x) (* x x)) '(1 2 3 4 5))

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