Official Aviation Definition
Density altitude is pressure altitude corrected for non-standard temperature. In hot and humid conditions, air molecules spread apart, reducing air density. High density altitude significantly degrades aerodynamic lift, engine thrust, and propeller efficiency.
In Simple Words
How high the airplane "feels" like it is flying based on hot weather and thin air. On hot summer days, a low-elevation runway can act like a high mountain airport.
Real-World Flight Example
At Phoenix Sky Harbor in 115°F summer heat, the field elevation is 1,135 feet MSL, but the density altitude can exceed 5,000 feet, requiring longer takeoff rolls.