What is a simple definition of pronation of the foot?

Pronation of the foot refers to the way the foot rolls inward to distribute a person's weight as he walks or runs, according to Runner's World. An inward roll of about 15 percent is normal and more than adequate to support the person's weight.

The arch of a person's foot affects the way the foot pronates, says Runner's World. A person who has flat feet often overpronates. This means the roll of the foot exceeds 15 percent. A person with a high arch underpronates or supinates. The foot rolls less than 15 percent, and the weight of the body isn't distributed evenly. Both overpronation and underpronation increase the risk of injury to the feet.