sit in

Short definition from Wiktionary

Verb

1. 1
   To attend a class, educational presentation, meeting, etc. for one time, as a visitor.
2. 2
   To take part in a group musical performance, especially a jazz session, as a temporary guest.
3. 3
   To participate in a sit-in protest.

Noun

1. 1
   A peaceful form of protest in which people occupy an area and refuse to leave.

Adjective

1. 1
   Designed to be sat in.