The Full Form of CTF is‍ Capture The Flag.

Capture the flag (CTF) is a traditional outdoor sport where two teams each have a flag (or other marker) and the objective is to capture the other team’s flag, located at the team’s “base,” and bring it safely back to their own base. Enemy players can be “tagged” by players in their home territory and, depending on the rules, they may be out of the game, become members of the opposite team, sent back to their own territory, or frozen in place (“in jail”) until freed by a member of their own team.