Flat feet: Absence of an arch in the sole of the foot that causes the foot to lie flat when the person is standing. All babies have flat feet because their arches are not yet built up (and their feet tend to be plump). This condition may persist into adulthood, or an arch may form as the child grows. Flat feet can also be acquired, as in jobs that require a great deal of walking and carrying of heavy objects. Also called pes planus.