Flatfoot deformity (pes planus) is a fallen arch of the foot, in which most or all of the sole of a person's foot touches the ground when standing. This lack of a properly developed arch can occur in one or both feet.
Flatfeet are common in infants and toddlers. Many young children have what is called "flexible flatfoot," where the arch is visible when the heel of the foot is raised, but then flattens when the child stands on the full foot. In most of these children, correct permanent curvature of the arch will form as they grow, after they learn to walk. Some children, however, may have what is called "rigid flatfoot," where the arch remains flat because of an additional bone or joint problem.
In most adults with flatfoot, the fallen arch evolves over time due to injury or other condition. This is called posterior tibial tendon dysfunction or "adult aquired flatfoot." For more on this condition, read the in-depth article listed below.