1. In mathematics, amplitude means size or magnitude. The origin of the word is the same Indo-European ple root that gives us "plus" and "complement." The more immediate Latin source is amplus for wide. Today, "amplitude" is used to describe, among other things, the distance a periodic function varies from its central value, and the magnitude of a complex number.
2. In physics, amplitude refers to the maximum departure of a cyclical quantity from its mean value, i.e., the maximum height of a wave peak or the maximum depth of a wave trough. The amplitude of a pendulum is half the extent of its swing. Amplitude modulation (AM) is a common method of encoding a carrier wave in radio.
3. One of the meanings of amplitude in astronomy is the difference between the maximum and minimum magnitudes of a variable star, i.e., the total range of its brightness.