In car handling, slip angle is the angle between a wheel's actual direction of travel and the direction towards which it is pointing. The more slip angle the more side force a wheel will generate - this is true up to a certain maximum, typically around 4-8 degrees, where the side force will start to drop off again.
A nonzero slip angle arises because the walls of a rubber tyre are flexible; hence the contact patch is deformed by the sideways forces during cornering.
If the front wheels of a car have greater slip angle than the back ones, this can be a factor in understeer. The opposite condition can contribute to oversteer. Since slip angles change with cornering acceleration, it is quite common for a vehicle to understeer when taking a corner at one speed, and oversteer when going around the same corner at a different (typically higher) speed.
