Technical term in logic and semantics referring to the set of all things a concept applies to. Contrasts with 内包 (intension/connotation).
The extension of 'dog' includes both Chihuahuas and Saint Bernards.
The distinction between extension and intension is fundamental in semantics.
A Sino-Japanese compound of 外 (outside, external) and 延 (extend, stretch), likely coined as a translation of the Western logical term 'extension'.