Definition of: infinitesimal

(in′fin·ə·tes′ə·məl) adjective
1. Infinitely small.
2. So small as to be incalculable and insignificant for all practical purposes.
3. Math. Denoting a quantity conceived as continually diminishing toward zero as a limit.
—noun An infinitesimal quantity. [<NL infinitesimus <infinitus infinite + -esimus (after centesimus hundredth)]
—in′fin·i·tes′i·mal·ly adverb