variable vs covariant

variable is a noun but covariant is not a noun.

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

variable (noun)	covariant (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

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