Real Dictionary

sit in in American English
to attend or take part as a visitor or temporary participant
to take part; participate; attend

sit-in
countable noun: A sit-in is a protest in which people go to a public place and stay there for a long time.

sit-in in American English
a method of protesting the policy of a government, business, etc., in which demonstrators sit in, and refuse to leave, a public place, thus blocking traffic, disrupting operations, etc.
noun: an organized passive protest in which the demonstrators occupy seats prohibited to them, as in restaurants and other public places

sit-in in British English
noun: a form of civil disobedience in which demonstrators occupy seats in a public place and refuse to move as a protest