Identities pl. : of Identity

Example:
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Taxonomy
identities, cosine, Abramowitz and Stegun, trigonometric functions, sum, formulae, tangent, product, variables, cot, derivative, sum and difference identities, values, the unit circle, List of trigonometric identities, radians, csc, cosine wave.

Defination from the web:
identities involving certain functions of one or more angles. ... These identities are useful whenever expressions involving trigonometric functions need to be simplified. ...