From Merriam-Webster Online
Main Entry: 4sear
Function: noun
Etymology: probably from Middle French serre grasp, from serrer to press, grasp, from Late Latin serare to bolt, latch, from Latin sera bar for fastening a door
Date: 1596
: the catch that holds the hammer of a gun's lock at cock or half cock
In many weapons it is a part of the trigger, in others it is separate and is moved by the trigger.
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SGB beat me too it.