The adjective colloquial means a word or a phrase that is casual and most often used in conversations.
Synonyms are informal, particular, vernacular, or popular.
The word origins in 1751 and it comes from colloquy meaning “a conversation” plus –al. It was used in 1752 as “peculiar or appropriate to the language of common speech or familiar conversation”, especially as distinguished from elegant or formal speech.
His speech was colloquial and simple.
The teacher asks his students to write a colloquial speech.
Some may think that colloquial language is not good.