noun, used as a prefix
Mathematics prefix meaning 'identical' or 'identity', used in terms like 恒等式 (identity equation) and 恒等変換 (identity transformation).
この式は恒等的に成り立つ。
This equation holds identically.
恒等写像はすべての要素を自分自身に写す。
The identity mapping maps every element to itself.
同一 is a general adjective/noun meaning 'the same' or 'identical', while 恒等 is a technical prefix used in mathematics for identity concepts.
Compound of 恒 (constant, permanent) and 等 (equal, same). The exact historical derivation as a mathematical term is uncertain, but it is used to translate the Western mathematical concept of identity.