A person or thing that prevents something from happening.
The new security system acts as a preventer of unauthorized access.
A device or mechanism designed to stop or control a process or action.
The blowout preventer is a critical safety device on an oil rig.
Nautical: a rope, chain, or tackle rigged to steady a spar or sail and prevent unwanted movement, especially to stop a boom from swinging when sailing downwind.
During the storm the crew rigged a preventer to keep the boom from jibing unexpectedly.