periodic function

Wiktionary

1. n. mathematics Any function whose value repeats after the regular addition of a period to its independent variable; i.e. f(x+t) = f(x) for some t over all x

GNU Webster's 1913

1. adj. (Math.) a function whose values recur at fixed intervals as the variable uniformly increases. The trigonomertic functions, as sin x, tan x, etc., are periodic functions. Exponential functions are also periodic, having an imaginary period, and the elliptic functions have not only a real but an imaginary period, and are hence called doubly periodic.