• A foot in which the arch of the instep is flattened so that the entire sole of the foot rests upon the ground; also, the deformity, usually congential, exhibited by such a foot; splayfoot.
flatfoot.
flat foot is a lack of the medial arch of the foot. The development of the medial arch occurs within 2-3 years of walking. In younger children the medial arch is largely obliterated by a fat pad.
Flat' foot` (Medicine) A foot in which the arch of the instep is flattened so that the entire sole of the foot rests upon the ground; also, the deformity, usually congential, exhibited by such a foot; splayfoot. 
Flat-foot is a deformity of the foot in which the arch sinks so that the whole extent of the sole is in contact with the ground.
Human foot in which the arch or arches specially the internal metatarsal arch, have either been broken due to some reason or have not been developed as in normal foot.