Informal speech or behaviour is relaxed and friendly rather than serious, very correct, or official.
relaxed , casual
She is refreshingly informal.
她的随和令人耳目一新。
An informal situation is one that is relaxed and friendly and not very serious or official.
The house has an informal atmosphere.
这屋子里有一种轻松随意的氛围。
This door leads to the informal living area.
这扇门通向气氛随和的会客区。
Informal clothes are casual and suitable for wearing when you are relaxing, but not on formal occasions.
For lunch, dress is informal.
吃午餐可以穿着休闲装。
You use informal to describe something that is done unofficially or casually without planning.
The two leaders will retire to Camp David for informal discussions.
这两位领导人将去戴维营进行非正式会谈。
This was an informal, unofficial investigation.
那是个随意的非官方调查。