Plain English Meaning
The most power the engine can give you at this moment, given where you are and what the air is like. It changes with altitude and temperature, so it is not always the same value.
Full Definition
The greatest amount of power the engine can produce at a given moment under the existing conditions of altitude, temperature, and engine configuration. It is the upper limit of what the powerplant can deliver right now, not a fixed number from the manual.
Context Anchor
Used in straight-and-level flight discussions when comparing how much power the airplane can produce with how much power it needs to maintain speed and altitude.
Derivation
Maximum comes from a Latin word meaning “greatest.” Available means “ready for use.” Together, the phrase points to the greatest power that is actually ready and usable in the present , not just a rated number on paper.