salt flat vs salt plain

salt flat and salt plain both are nouns.

Word	Noun	Adjective	Verb	Adverb
salt flat	Yes	No	No	No
salt plain	Yes	No	No	No

As nouns, salt flat and salt plain are synonyms defined as:

salt flat and salt plain: a flat expanse of salt left by the evaporation of a body of salt water

salt flat (noun)	salt plain (noun)
a flat expanse of salt left by the evaporation of a body of salt water	a flat expanse of salt left by the evaporation of a body of salt water