An idiom or fixed phrase used commonly in a language; often whose meaning isn't deducible from the literal meanings of its parts.
英語の慣用句は直訳すると意味が通じないことが多い。
English idioms often don't make sense when translated literally.
『頭が切れる』は日本語の慣用句で、字義どおりの意味ではない。
'Atama ga kireru' is a Japanese idiom and doesn't mean what the words literally say.
慣用句を覚えると、会話がずっと自然になります。
Learning idioms makes your conversation sound much more natural.