Definition from Wiktionary, the free dictionary
- 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?
- (mathematics) Of or pertaining to values that approach zero as a limit.
- (informal) Very small.
- 2001 — Eoin Colfer, Artemis Fowl, p 221
- Then you could say that the doorway exploded. But the particular verb doesn't do the action justice. Rather, it shattered into infinitesimal pieces.
Usage notes 
- Strictly, this adjective, like infinite, is incomparable, so more infinitesimal and most infinitesimal are proscribed, especially in the mathematical sense. However, these forms do occur in informal usage, where the very small (but measurable) sense is most common.
- See also Wikisaurus:tiny
- (mathematics) A non-zero quantity whose magnitude is smaller than any positive number (by definition it is not a real number).
A non-zero quantity whose magnitude is smaller than any positive number