A chirp is a signal in which the frequency increases ('up-chirp') or decreases ('down-chirp') with time. It is commonly used in sonar and radar, but has other applications.
In a linear chirp, the frequency varies linearly with time:
f(t) = f0 + f×k×t
This type is easy to generate.
In a geometric chirp, the period of the signal varies linearly with time. Although harder to generate, this type does not suffer from reduction in correlation gain if Doppler shifted by a moving target.