By "party", I mean a physical social gathering of people for the purposes of having fun. It may be used in a sentence as "I am throwing a party!" or "Let's party!".
Basically what I am trying to say is the default "party".
I've never been to any, and I have no idea how people spend their time on parties, so I am curious how you did.
It's a social gathering. Of a specific group of people. The difference between a party, and a group lunch, is basically the time frames, and a more curated group of people tend to go to a party. Whereas a lunch tends to be who's available.
Parties tend to be some celebration, which is a human social norm, but really the reason for the party is less important than the social interaction. Humans need periodic social contact to maintain relationships, and parties are a good avenue for that.
There's a whole spectrum of parties, there's dance parties, there's drug parties, there's alcohol parties, there's board game parties, there's beer parties, there's dinner parties, there's anything. Any form of human social engagement could be a party.
Just think of parties as a way to maintain your social standing, and refresh relationship status, with a group of related people at once. So it's very efficient in that regard.
Good description.
A party is also simply an opportunity for people to get together, to hang out together, to catch up on what the other people have done or experienced recently, and to tell/brag/complain about your own life.
This is how you maintain friendships: keep in touch, have a half-decent understanding of what their life is like, and share their ups&downs as well as your own.