| Infinitesimal 1. (a.) Infinitely or indefinitely small; less than any assignable quantity or value; very small. 2. (n.) An infinitely small quantity; that which is less than any assignable quantity. Thesaurus Entries atomic, corpuscular, embryonic, evanescent, germinal, granular, impalpable, imperceptible, imponderable, inappreciable, indiscernible, intangible, invisible, microcosmic, microscopic, molecular, subatomic, tenuous, thin, ultramicroscopic, unseeable |