Expressions used in spoken language; informal or conversational phrasing as opposed to written/formal style.
この文章は口語表現が多くて、話し言葉に近い印象を受ける。
This text has many colloquial expressions and gives a conversational impression.
試験では口語表現よりも文語的な表現が求められることがある。
On some exams, more formal/written expressions rather than colloquial ones may be required.
友達には口語表現で話してもいいが、ビジネスメールでは避けるべきだ。
It's fine to speak colloquially with friends, but you should avoid it in business emails.