You have two options, depending on how technical you want to go.
A mass noun or non-count noun refers to a quantity that is treated as a composite unit. One cannot ordinarily modify the mass noun with a numeral without a unit of measurement. For instance, rather than referring to four sands or ten rices (a numeral and a plural noun), we just refer to sand or rice. If you add some rice, then you have more rice, but that addition is only countable using some other unit - grains, grams, ounces. The contrast would be a count noun.
A related property you are appealing to is called cumulativity in linguistics (Wikipedia):
In linguistic semantics, an expression X is said to have cumulative reference if and only if the following holds: If X is true of both of a and b, then it is also true of the combination of a and b. Example: If two separate entities can be said to be "water", then combining them into one entity will yield more "water". If two separate entities can be said to be "a house", their combination cannot be said to be "a house". Hence, "water" has cumulative reference, while the expression "a house" does not.
So mass nouns like water use cumulative reference: add water to water and you have water (1+1=1). So, incidentally, do plurals: if you add houses to houses, then the result is still houses (plural+plural=plural).
The contrast is quantization or quantized reference (Wikipedia; see also Fleischhauer and Gabrovska, 103), where you have one banana but the form changes upon counting (two bananas). Another way to think of it is trying to divide an apple versus dividing water. There is no subpart of apple that is a whole apple (1 divided by 2 is a half), but you can divide water to get subparts of water (mass divided by half is still mass). The apple is quantized; water isn't.
Note that the latter terms (cumulative, quantized) are quite technical and not commonly known. Mass nouns and count nouns are at least more familiar terms to someone who has gone through a grammar class.