A sear is the part of the trigger mechanism that holds the hammer back until the correct amount of pressure has been applied to the trigger. On open bolt firearms, it is used to hold back the bolt.
On an automatic sear, the sear holds the hammer until the correct amount of pressure has been applied to the sear itself, usually by way of the bolt carrier, at which point the sear will release the hammer. This has the dual effect of controlling the rate of fire, as well as ensuring that the hammer does not follow the action back into battery, causing a failure to ignite or a slamfire (unintentional discharge caused by a malfunction).
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