infinitesimal

Short definition from Wiktionary

Adjective

1. 1
   Incalculably, exceedingly, or immeasurably minute; vanishingly small.
2. 2
   Of or pertaining to non-zero quantities whose magnitude is less than any positive rational number.
3. 3
   Very small.

Noun

1. 1
   A non-zero quantity whose magnitude is smaller than any positive number (by definition it is not a real number).