(1) Of or pertaining to something that cannot be dissolved, or solved.
(2) Having the quality of being difficult to solve or explain; insolvable.
Insoluble is often applied to compounds that are poorly soluble or do not readily dissolve in a particular solvent; e.g. oil is insoluble in water.
Word origin: insolubilis : in-, not + solvere, to loosen.
Related forms: insolubility (noun), insolubly (adverb).
Related terms: insoluble soap.