infinitesimal

infinitesimal \in-fin-i-TES-uh-muhl\, adjective:

1. indefinitely or exceedingly small; minute: infinitesimal vessels in the circulatory system.
2. immeasurably small; less than an assignable quantity: to an infinitesimal degree.
3. of, pertaining to, or involving infinitesimals.

Then the waves increased in strength, and sought to improve his understanding, reconciling him to the multiform entity of which his present fragment was an infinitesimal part.
-- H.P. Lovecraft and E. Hoffman Price, "Through the Gates of the Silver Key," Weird Tales, July 1934

I knew that her diversions described an arc only minutely different from before; but that infinitesimal alteration separated our steps, one from another, with brutal absoluteness.
-- Claire Messud, The Last Life, 1999

Infinitesimal comes from the Latin word infinītus meaning "boundless." The suffix -ēsimus was added to ordinal numbers in Latin.