Real Dictionary

sit down
phrasal verb: If you sit down and do something, you spend time and effort doing it in order to try to achieve something.
singular noun: If you have a sit-down, you sit down and rest for a short time.

sit down in American English
to take a seat
adjective: done or accomplished while sitting down
noun: a period or instance of sitting, as to relax, talk, or the like

sit down in British English
to adopt or cause (oneself or another) to adopt a sitting posture
noun: an instance of sitting down

sit-down in British English
a strike, usually illegal, in which the strikers stay inside a factory, etc., refusing to work or leave until agreement is reached
served to persons, as guests, who are seated at a table
to lower oneself to a sitting position; take a seat
modifier: of or relating to an event where people sit down
noun: a seat; an act of sitting down