What is a mathematical statement that equates one quantity with another called?

1. An identity is a mathematical statement that equates one quantity with another. 2. Trigonometric identities let an expression be rewritten with sine and cosine ratios only. 3. That rewriting is what makes it possible to prove further identities and solve equations. _Source: Siyavula Mathematics Grade 11 (Everything Maths, CC BY 4.0), Chapter 6: Trigonometry_