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 is derived from the root infinite, which usually conjures up thoughts of immeasurably large quantities or sizes. But in this case, infinite connotes something immeasurably small.

Definitions of infinitesimal
  1. adjective
    infinitely or immeasurably small
    synonyms: minute
    little, small
    limited or below average in number or quantity or magnitude or extent
  2. noun
    (mathematics) a variable that has zero as its limit
    see moresee less
    type of:
    variable, variable quantity
    a quantity that can assume any of a set of values
Word Family

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