Plain English Meaning
It is the way large airliners and similar aircraft are required to perform during every phase of flight, with strict rules so the airplane can still fly safely even if an engine fails at the worst possible moment.
Full Definition
The set of takeoff, climb, cruise, and landing performance standards that apply to aircraft certificated under the transport category rules of 14 CFR Part 25. These standards are more demanding than those for normal or utility category aircraft and include requirements such as guaranteed climb performance with one engine inoperative, defined takeoff speeds (V1, VR, V2), accelerate-stop and accelerate-go distances, and certified takeoff and lengths.