na-adjective
Describes something that is done or expressed according to established custom or common practice, often with a nuance of being conventional or idiomatic rather than literal.
この表現は慣用的な言い回しです。
This expression is an idiomatic phrase.
慣用的な手
Please handle it according to the customary procedure.
慣例的 also means 'customary' but emphasizes established precedent or convention, while 慣用的 often refers to linguistic or practical common usage.
Derived from 慣用 (common usage, idiom) + 的 (adjectival suffix). The exact historical derivation is straightforward; the word follows a regular pattern of forming adjectival nouns from nouns.