Official Aviation Definition
A runway is a defined rectangular strip on a land aerodrome designated and prepared for aircraft takeoffs and landings. Runways are numbered by their magnetic compass heading rounded to the nearest 10 degrees; for example, a runway aligned with 092 degrees is numbered Runway 09.
In Simple Words
The paved strip of asphalt or concrete where airplanes take off and land.
Real-World Flight Example
Runway 28L at San Francisco International Airport has a magnetic orientation of 284 degrees and a length of 11,381 feet.