A meeting up or gathering; coming together at a designated place/time (can be used as a noun or as the verb 'to gather').
集合時間は午前9時、駅の北口に集合してください。
Meeting time is 9:00 AM; please gather at the north exit of the station.
遠足の前に全員が校庭に集合した。
Everyone gathered on the school grounds before the excursion.
緊急時には指定の避難場所に集合すること。
In an emergency, assemble at the designated evacuation site.
In mathematics, a 'set' — a collection of elements; the concept of a set and its members/subsets.
自然数の集合をNで表すことが多い。
It's common to denote the set of natural numbers by N.
この問題では二つの集合の共通部分を求めなさい。
In this problem, find the intersection of the two sets.