Blocker (noun)

1. A person or thing that blocks or obstructs something.
2. A defensive player in American football positioned to stop the opposing team's running play.

Origin:

Mid 19th century: from block + -er.

Examples:

1. He put his hand up as a blocker to shield his eyes from the sun.
2. The blocker on the offensive line protected the quarterback.
3. The blocker in the doorway wouldn't let anyone enter the room.
4. The software had a built-in blocker to prevent unwanted pop-ups.
5. The blocker on the basketball team was tasked with stopping the other team's star player.