flatfoot vs pes planus

flatfoot and pes planus both are nouns.

Word	Noun	Adjective	Verb	Adverb
flatfoot	Yes	No	No	No
pes planus	Yes	No	No	No

As nouns, flatfoot and pes planus are synonyms defined as:

flatfoot and pes planus: a foot afflicted with a fallen arch; abnormally flattened and spread out

Other synonyms of flatfoot include splayfoot.

flatfoot (noun)	pes planus (noun)
a foot afflicted with a fallen arch; abnormally flattened and spread out	a foot afflicted with a fallen arch; abnormally flattened and spread out
a policeman who patrols a given region