free safety 
noun American Football
A defensive back who is usually free from an assignment to cover a particular player on the opposing team.He has played everything from strong safety to free safety to nickel linebacker to nickel corner....
- He is listed as a strong safety but also lines up at linebacker, cornerback, free safety and at the line of scrimmage.
- I always tried to be in a place where the quarterback didn't expect to find a free safety.