From Japanese 用言.
용언 (yong-eon, hanja 用言)
- (grammar) a word that can be conjugated
Usage notes 
Words in the 용언 (yong-eon) class are those that can be conjugated and used as Korean predicates. The class includes 동사 (dongsa, “verbs”), 형용사 (hyeong-yongsa, “adjectives”), the copula 이다 (ida), and its antonym 아니다 (anida).