Plain English Meaning
It's the drag caused by air sliding along the outside of the airplane. The air closest to the surface gets slowed down by rubbing against it, and that rubbing is a force that pulls back on the aircraft.
Full Definition
The drag force produced by air rubbing against the surface of an aircraft as it flows past. It arises within the boundary layer, where the air in direct contact with the skin is slowed by viscosity, creating a tangential resisting force along every exposed surface.
Context Anchor
Seen in high-speed flight and boundary layer discussions when explaining how surface condition and airflow affect drag.
Derivation
Skin' here means the outer surface of the aircraft, and 'friction' comes from the Latin 'frictio,' meaning rubbing. Together it literally describes air rubbing against the aircraft's skin.