Plain English Meaning
The slowing effect created by air sliding across the surface of the aircraft. The rougher the surface, the more it slows the airplane down.
Full Definition
The resistance produced when air flows across the surface of an aircraft and rubs against the skin, creating drag along the boundary between the air and the airframe. Surface roughness — including ice, frost, or contamination — increases this resistance and reduces aerodynamic efficiency.
Context Anchor
Seen in icing discussions, where ice roughens the wings and other surfaces and makes the airplane harder to move through the air.
Derivation
From Latin frictio, meaning 'a rubbing.' Frictional drag is literally the drag caused by air rubbing against the aircraft's surface.