covariant vs variable

covariant is not a noun while variable is a noun.

Word	Noun	Adjective	Verb	Adverb
covariant	No	Yes	No	No
variable	Yes	Yes	No	No

covariant (noun)	variable (noun)
	a quantity that can assume any of a set of values
	a symbol (like x or y) that is used in mathematical or logical expressions to represent a variable quantity
	something that is likely to vary; something that is subject to variation
	a star that varies noticeably in brightness

covariant (adjective)	variable (adjective)
changing so that interrelations with another variable quantity or set of quantities remain unchanged	liable to or capable of change
	(used of a device) designed so that a property (as e.g. light) can be varied
	marked by diversity or difference