a. [ Cf. F. infinitésimal, fr. infinitésime infinitely small, fr. L. infinitus. See Infinite, a. ] Infinitely or indefinitely small; less than any assignable quantity or value; very small. [ 1913 Webster ]
Infinitesimal calculus,
n. (Math.) An infinitely small quantity; that which is less than any assignable quantity. [ 1913 Webster ]
adv. By infinitesimals; in infinitely small quantities; in an infinitesimal degree. [ 1913 Webster ]