Official Aviation Definition
A crosswind is any wind that has a component blowing perpendicular to the runway centerline or aircraft flight path. Pilots must calculate the crosswind component before landing to ensure it does not exceed the aircraft maximum demonstrated crosswind limit.
In Simple Words
A wind blowing sideways across the runway, trying to blow the airplane off center during takeoff or landing.
Real-World Flight Example
With runway 09 facing 090 degrees and winds from 120 at 20 knots, the crosswind component is 10 knots from the right.