in·fin·i·tes·i·mal 
(ĭn′fĭn-ĭ-tĕsə-məl)
(ĭn′fĭn-ĭ-tĕsə-məl)adj.
1. Immeasurably or incalculably minute.
2. Mathematics Capable of having values approaching zero as a limit.
n.
1. An immeasurably or incalculably minute amount or quantity.
2. Mathematics A function or variable continuously approaching zero as a limit.
[From New Latin īnfīnītēsimus, infinite in rank, from Latin īnfīnītus, infinite; see INFINITE.]
in′fin·i·tesi·mal·ly adv.