infinitesimal /ˌɪnfɪnɪˈtɛsɪməl/adjective 3noun 2forms 3 adjective3 •Incalculably, exceedingly, or immeasurably minute; vanishingly small.Do you ever get the feeling that you are but an infinitesimal speck, swallowed by the vastness of the universe and beyond? •Very small. •Of or pertaining to non-zero quantities whose magnitude is less than any positive rational number. noun2 •A non-zero quantity whose magnitude is smaller than any positive number (by definition it is not a real number). •Something infinitesimally small. formsinfinitesimals · more infinitesimal · most infinitesimal