identity

user=> (identity 4)
4

user=> (filter identity [1 2 3 nil 4 false true 1234])
(1 2 3 4 true 1234)

user=> (map #(%1 %2) (cycle [inc identity]) [1 2 3 4 5 6 7 8 9 10])
(2 2 4 4 6 6 8 8 10 10)

user=> (partition-by identity (sort "abcdaabccc"))
((\a \a \a) (\b \b) (\c \c \c \c) (\d))

user=> (map first (partition-by identity [1 1 2 3 3 1 1 5 5]))
(1 2 3 1 5)

user=> (group-by identity "abracadabra")
{\a [\a \a \a \a \a], \b [\b \b], \r [\r \r], \c [\c], \d [\d]}

user=> (map #(identity %) [1 2 3 4]) ; ~ (map (fn [x] x) [1 2 3 4])
(1 2 3 4)

; `identity` can serve in workarounds, because you can't pass a macro
; to a function. For example, you can't pass `and` as a parameter to `apply`:
(apply and '(true 1 "yes"))
; \=> CompilerException... Can't take value of a macro...

; Instead:
(every? identity '(true 1 "yes"))