Plain English Meaning
What the runway is made of and how it slopes. A soft, wet, or rough surface makes the aircraft accelerate or stop differently than a smooth dry one. A runway that tilts uphill or downhill also changes how much room the aircraft needs.
Full Definition
Two physical characteristics of a runway that affect takeoff and landing performance: the surface (the material the runway is made of and its current condition, such as dry asphalt, grass, gravel, or a wet or contaminated surface) and the gradient (the slope of the runway, expressed as a percentage representing the change in elevation from one end to the other). Both factors alter the distance an aircraft needs to take off or land and must be considered during performance planning.