infinitesimal(noun, adjective) adjective1. infinitely or immeasurably smallSimilar word(s): little, small, minuteSentences with infinitesimal as an adjective:- Do you ever get the feeling that you are but an infinitesimal speck, swallowed by the vastness of the universe and beyond? noun1. (mathematics) a variable that has zero as its limitDefinition categories: thought, variable