collaborator

Concise Oxford English Dictionary © 2008 Oxford University Press:
collaborate /kəˈlabəreɪt/

▶verb

• 1 work jointly on an activity or project.

• 2 cooperate traitorously with an enemy.

– derivatives
collaboration noun,
collaborationist noun & adjective,
collaborative adjective,
collaboratively adverb,
collaborator noun.

– origin C19: from L. collaborat-, collaborare ‘work together’.