We call the quantity of more than one (singular) plural. Is there any general word similar to this for a quantity less than one (singular)?
In particular, in terms of grammatical number, English has only singular for when there is 1 or -1 of something, and plural for all other numbers.
Some other languages have forms for particular numbers other than those, and indeed English shows a trace of a dual number in words like both and some ways of treating neither, but only a trace as phrases using them will still be otherwise using just singular and plural.
Outside of grammar, for numbers between zero and one specifically, we have fractional. There are also several terms equivalent in form to singular that are more specific about the number (dual, trial, quadral).