When it's incalculably small, when teensy seems large compared to it, then it is infinitesimal. "The equipment was so sensitive that even the infinitesimal dust molecules that swept in with the open door altered the readings."
Infinitesimal is such an extreme description of a microscopic something that the word is used as an exaggeration more often than to describe something actually quite small. "The chances of Roxy going out on a date with Norbert were infinitesimal. She barely even knew he existed." Infinitesimal has as its root infinite which usually conjures up thoughts of immeasurably large quantities or sizes. In this case infinite refers to something immeasurably small.
