The prefix co- means “together” or “with.” When you see it at the start of a word, it signals that multiple people or things are doing something jointly or sharing a role.

A co-author writes a book with someone else. Co-workers work at the same place. When you cooperate with classmates on a group project, you're operating together toward a shared goal. A co-pilot sits alongside the main pilot, ready to help fly the plane.

Sometimes the prefix attaches directly to the root word, like coexist (to exist together in the same space). Other times there's a hyphen, especially with proper nouns or to avoid confusion, like co-founder or co-owner.

You'll find it everywhere: in science (coefficient), in business (co-manager), and in everyday life (co-captain of a team). When you see “co-” you know immediately that whatever follows involves partnership, collaboration, or shared responsibility. It's one of the most useful prefixes in English because humans do so many things together.