### Video Summary:

Many things in daily life repeat with a predictable pattern. Because sine and cosine repeat their values over and over in a regular pattern, they are examples of periodic functions. Periodic Function
A periodic function is a function f such that
f (x) = f (x + np),
for every real number x in the domain of f , every integer n, and some positive real number p. The smallest possible positive value of p is the period of the function.