great gathering; large assembly
Used for a large-scale gathering of people, things, or characters, often with a sense of excitement or comprehensiveness.
An event was held where popular characters gathered in large numbers.
Fans from all over the country gathered in great numbers at the venue.
Popular anime characters all gathered together at the venue.
An event featuring a huge collection of popular snacks was held.
集結 is a more formal/military-sounding word for forces or people massing together, whereas 大集合 has a lighter, often festive nuance used for people, characters, or products.
集会 refers to an organized meeting or rally with a purpose, while 大集合 simply emphasizes the large size of a gathering without implying an agenda.
Compound of 大 (だい, 'big, great') and 集合 (しゅうごう, 'gathering, assembly').