The smaller, more intimate continuation of a party after the club closes, often stretching into the next day.
"Afters" is not round two of the club. That's the misconception, and it's the one that gets people un-invited. The main room runs on volume and momentum; the afters runs on a different tempo — smaller, dimmer, closer, the music at a level where people can actually talk. Some are official (a promoter's second venue, a legal after-hours club) and some are somebody's living room at 7am with a phone plugged into a speaker. Both are "the afters." The difference between them is mostly how many people you're allowed to know.
Which is the part outsiders miss: the afters is an invitation, not a location. Getting told the address is a small act of trust, and the etiquette follows from that. Don't bring three strangers you met at the bar. Don't be the person who arrives loud and cranks everything back up to peak-time energy — you'll read the room wrong and the room will remember. Don't overstay the host's goodwill. In British and Irish usage the word carries its older baggage of the illicit pub lock-in, and that flavour survives: an afters is a little bit private, a little bit past the official end.
In practice it's where the comedown gets social instead of lonely — the cuddle-puddle end of the night, the slow set, the people you actually wanted to keep talking to once the crowd thinned out. Treat it as the intimate encore, not the second headline act, and you'll keep getting invited to it.