Plain English Meaning
It is the altitude the airplane and engine actually feel, based on how thin the air is right now. Hot, high, or humid conditions make the air thinner, so the airplane performs as if it were at a higher altitude than the runway sign says.
Full Definition
Pressure altitude corrected for non-standard temperature. It is the altitude at which the standard atmosphere would have the same air density as the air the aircraft is currently flying in. Density-altitude rises as temperature, humidity, or field elevation increases, and it directly affects engine power, , and produced by .