means A social gathering held for fun and celebration, or alternatively an organized group of people united by a shared cause, role, or legal status.

from From Old French 'partie,' meaning a part, portion, or side, itself from Latin 'partire,' to divide or share out — the same root that gives us 'part' and 'partition.' The 'group of people' sense (a political party, a search party, the guilty party) came first, naming a faction or 'side' split off from a whole. The 'celebration' sense is the cheerful latecomer, the idea being a part of society gathered together for a shared occasion.