A counter-terrorist plan or group is one whose job is to stop violent political groups before they attack, and to weaken the people who carry those attacks out. In real English this word is most often an adjective: a counter-terrorist unit, counter-terrorist operations, counter-terrorist police. As a noun, a counter-terrorist can be a person in that job, or, less often, the group itself. You meet it in news, army, and police English. Though actually, everyday speakers mix it with counter-terrorism and nobody in the street minds much. In careful writing they are not twins: counter-terrorism is the work and the policy; counter-terrorist usually describes the people, teams, and actions that do that work. It is not a kind of terrorist.