amongst mathematicians, are defined to be infinitely small quantities.

In the method of infinitesimals, the element, by which any quantity increases or decreases, is supposed to be infinitely small, and is generally expressed by two or more terms, some of which are infinitely less than the rest; and this being neglected as of no importance, the remaining terms form what is called the difference of the proposed quantity. The terms which are neglected in this manner, as infinitely less than the other terms of the element, are the very same which arise in consequence of the acceleration or retardation of the generating motion, during the infinitely small time in which the element is generated; so that the remaining terms express the elements which would have been produced in that time, if the generating motion had continued uniform; and therefore those differences are accurately in the same ratio to each other as the generating motions or fluxions. Hence, though in this method infinitesimal parts of the elements are neglected, the conclusions are accurately true without even an infinitely small error, and agree precisely with those which are deduced by the method by fluxions. See Fluxions.