While attempting to answer this question @ProgramFox has been trying to come up with an example of a noun with no plural.
The best I've come up with is egotism and Judaism. (Fluxmay be a viable contender as there is a related question asking about its plural!)
Some of the contenders I've already shot down are air, water, and money. (Airs, waters, and monies are all legitimate plurals.)
My question is, a) Do my two examples have actual plurals? b) Are there any other non-ism nouns that don't have a plural?