παρείσακτος/pareisaktos

This word describes someone. It is an adjective.

This word can describe a person who has joined a group pretending to be like the other members. However, he really wants to harm or destroy them. This person is a spy or a traitor.
This word can describe a person whom another person has secretly brought into a group to harm or destroy it.

See: Galatians 2:4