Understeer, in an automobile, is when the front tires have a loss of traction during a cornering situation, thus, causing the front of the car to head towards the outside of the corner. Front wheel drive cars are prone to understeer. Understeer is basically a skid where you are turning the steering wheel, but the car keeps going straight, or is not turning as quickly as you would like. As the tires approach their traction limits, they tend to slip sideways across the road. If the front tires approach the traction limit more rapidly, the effect is that the front of the car takes a wider radius curve than the driver intended, the car is said to understeer. Understeer is safer and it is sometimes desired for better stablity while turning especially front wheel drive vehicles.
A more precise definition for Understeer is that the front tires are running at a greater slip angle than the rear wheels. This does not have to result in a loss of control of the vehicle. Only at the extreme edges of the vehicles performance envelope will the characteristics described above be observed.
The slip angle of a tire is defined as the difference between the direction of travel of the tire and the direction in which the tire is pointed.