A point in the signal path that allows connection of an external device. Typical connections are via a single 1/4" TRS jack which function as output AND input connections. The connector carries both the signal being sent from the mixer as well as the signal returning into the mixer. For example, on a mixer there may be an insert jack on each input channel. You might use the insert jack on a particularly hot (loud, explosive) vocal to connect a compressor. Other devices that are commonly used with insert jacks are reverb processors and equalizers.