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Biodiversity

Genetic variation, differences within a species

Magnetic variation, difference between magnetic north and true north, measured as an angle

Variation (astronomy), any perturbation of the mean motion or orbit of a planet or satellite, particularly of the Moon

Bounded variation

Calculus of variations, a field of mathematics which deals with functions of functions.

Variance (variation is a misnomer though), a measure of the spread of a variable in statistics

Variation (ballet), solo dance or dance figure

Variation (music), formal technique where material is altered during repetition

Variation (game), modifications made to a game by a community of players (as opposed to a central authority)