Having the capacity of varying or changing; capable of alternation in any manner; changeable; as, variable winds or seasons; a variable quantity.(adjective)
source: webster1913
noun (n)
something that is likely to vary; something that is subject to variation(noun.object)
Example: The weather is one variable to be considered.
source: wordnet30
a quantity that can assume any of a set of values(noun.cognition)