Plain English Meaning
The most power the manufacturer allows you to use for takeoff. In a go-around, you push the throttle up to this setting to climb away safely.
Full Definition
The highest engine power setting the manufacturer permits for takeoff, as specified in the Pilot's Operating Handbook (POH) or Airplane Flight Manual (AFM). It represents the upper limit of power the engine is certified to produce for the takeoff phase, and is the power setting applied during a go-around when full performance is needed.
Context Anchor
Seen in go-around procedures, when the pilot adds power to stop the descent and climb away from the runway.