An abbreviation for Automatic Gain Control. AGC circuits do just what it sounds like; they automatically adjust the level of incoming audio so it can be recorded and/or mixed properly in whatever the device is. The way they work is almost exactly like a typical compressor and the end result often sounds the same. They have most often been employed in low cost audio and video recorders to both avoid the expense of costly knobs and pots, and so equipment is less complicated to use. We pro’s always want access to our levels, but consumers often find it an unnecessary complication (this is sort of the same as the automatic versus manual transmission debate).