na-adjective
Describes something done or held by a group as a whole, or a property of a group rather than individuals.
これは集合的な決定です。
This is a collective decision.
集合的な責任を感じる。
I feel a collective responsibility.
集団的 is similar but often used for organized groups or teams, whereas 集合的 is more abstract and mathematical.
From 集合 (しゅうごう, 'collection, gathering') + 的 (てき, '-like, -al'). The suffix 的 forms adjectival nouns, often corresponding to English '-ive' or '-al'.