An Etymological Dictionary of Astronomy and Astrophysics

General:The greatness, size, or extent of something.
In any periodically varying function, the maximum absolute value of the quantity.
The magnitude range of a variable star.

Etymology (EN): L. amplitudo “wide extent, width,” from amplus “large”.

Etymology (PE): Dâmané “the foot or skirt of a mountain,” from dâman “skirt.”