Infinitesimal means infinitely small, approach zero in the limit. It is a modern concept that is a byproduct of the acceptance of the concept of infinity itself, which the Ancient Greeks and Ancient Romans mistakenly refused to recognize.
Reflecting its belated acceptance, the word itself did not exist in the English language until 1706. See Best New Conservative Words.
Like other aspects infinity, the concept of infinitesimal offers a powerful way to solve problems and even handle stress. Calculus advanced largely through an understanding and use of the infinitesimal.
The often-scarce resource of time, for example, becomes infinitely abundant if divided into infinitesimal segments.
Jesus introduced the concept of infinite time, or eternity, and infinitesimal time follows from that.