mag·ni·tude
/ˈmaɡnəˌto͞od/
noun
The degree of brightness of a star. The magnitude of an astronomical
object is now reckoned as the negative logarithm of the brightness; a
decrease of one magnitude represents an increase in brightness of 2.512
times. A star with an apparent magnitude of six is barely visible to the
naked eye.
Source: Cindy McKean, Oxford