collective
Involving a group of people acting or sharing together.
When people describe something as collective, they mean it belongs to or involves a whole group working together. A collective effort means everyone worked together toward the same goal. When your class earns a collective reward because the entire group behaved well all week, that reward came from everyone's combined actions.
The word emphasizes shared ownership or shared responsibility. A neighborhood might make a collective decision about where to build a new playground, meaning everyone had a say in the choice. Scientists talk about collective knowledge, referring to everything humanity has learned together over time, information no single person could possibly know alone.
You'll also hear collective used as a noun to describe a group organized to work together. A farmers' collective might share equipment and help harvest each other's crops. An artists' collective might work together in a shared studio space.
The key idea is that something collective involves many people acting as one unit. When a team wins a championship, that victory is collective: it belongs to every player, coach, and supporter together.