Infinite, as you probably know, means “endless” or “extending indefinitely.” It is ultimately from Latin infīnītus, the opposite of fīnītus, meaning “finite.” The notion of smallness in infinitesimal derives from the mathematical concept that a quantity can be divided endlessly; no matter how small, it can be subdivided into yet smaller fractions, or infinitesimals. The concept was still in its infancy in the early 1700s when Irish philosopher George Berkeley observed that some people “assert there are infinitesimals of infinitesimals of infinitesimals, etc., without ever coming to an end.” He used the word in a mathematical sense, too, referring to “infinitesimal parts of finite lines.” Later, the adjectival form acquired a general sense applicable to anything too small to be measured.

Examples

Stella includes a lottery ticket in every birthday card she sends despite the infinitesimal chances that it will be a winning one.

“In some jigsaws, every piece is unique in its contours. In others, those thousand pieces come in just a few basic shapes with only infinitesimal differences between them.” — Michael Dobie, Newsday (Long Island, New York), 10 May 2020