Slip angle
Slip angle is the difference between the direction a tire is pointed and the direction it is actually traveling while cornering.
Also called: tire slip angle
A cornering tire does not go exactly where it is pointed. The rubber in the contact patch twists and deforms, so the tire travels along a slightly different path than the wheel is aimed. The angle between the two is the slip angle, and it is how a tire generates cornering force.
As slip angle grows from zero, grip rises steeply, peaks, and then slowly falls away. For many performance tires that peak sits somewhere in the mid-to-high single digits of degrees, and it varies with the tire. Past it, adding more steering produces more noise and heat, not more grip.
This is why understeer and oversteer exist. If the fronts run past their peak slip angle first, the car pushes wide. If the rears do, it oversteers. Skilled drivers learn to feel the tire hovering at its peak, the edge where it is working hardest but not yet sliding away.
Driving at the tires' peak slip angle, and no further, is the essence of using all the grip available.
Shape of a typical tire curve (simplified). Force builds with slip angle, peaks (here near 5.5°), then falls away: that's the limit you feel as the car starts to slide.