In the Collins Cobuild English Usage Dictionary, n-var is used as an abbreviation for 'variable noun'
And the term 'variable noun' is explained in the following way:
Variable nouns are nouns that combine the behaviour of countable and uncountable nouns. They are like countable nouns when they refer to an instance or more than one instance of something, for example an injustice; injustices or to individual members of a class, for example a cake; cakes. Otherwise they behave like uncountable nouns, referring to something in more general terms [...].