An output that carries enough amplification to drive a set of headphones. The key word here is “amplification.” A headphone out should not be routed into a mic-level input. A headphone out will also very likely severely overload (and possibly damage) a line-level input. We’ve seen headphone outputs with watts of output power — enough to drive a speaker cabinet. Use these types of outputs for headphones and speakers, not to route into the inputs of other pieces of gear.