the ability to protect against attack or harm, or something used to protect against attack or harm:

A defense is also an argument in support of something, esp. when it has been criticized:

law The defense is the person or people in a court who have been accused of doing something wrong, and their lawyer:

(in sports) the ability to prevent the opposition from scoring points in a competition, or, esp. in football, the team without the ball that is trying to prevent its opposition from scoring points:

→  defence