You might recognize the word “infinite” at the start of this long word, but it’s not used to refer to unendingly long numbers. Instead, “infinitesimal” is a description of very small size. There may be an infinite number of fish in the ocean, but a single speck of plankton is infinitesimal. In this sense, it’s used as an adjective, but mathematicians also use “infinitesimal” as a noun meaning “a value approaching zero.” This usage is specifically applicable in calculus.