名詞 infinitesimal:

1. (mathematics) a variable that has zero as its limit

形容詞 infinitesimal:

1. infinitely or immeasurably small
   同義詞：minute, microscopic

1. Being neither infinite nor infinitesimal.
   非極限的，非無限小的既非無窮大也非無限小

in.fin.i.tes.i.mal

adj.（形容詞）
Immeasurably or incalculably minute.
極微小的，微不足道的
Mathematics Capable of having values approaching zero as a limit.
【數(shù)學(xué)】 近零的，極小的：無窮小地接近于零的
n.（名詞）
An immeasurably or incalculably minute amount or quantity.
無窮小的數(shù)量：非常小，幾乎無法測量的數(shù)量
Mathematics A function or variable continuously approaching zero as a limit.
【數(shù)學(xué)】 近零：一個(gè)接近于零的函數(shù)或者變量

來源：
From New Latin onfonotôsimus [infinite in rank]
源自 現(xiàn)代拉丁語 onfonotôsimus [在等級(jí)上無窮盡]
from Latin onfonotus [infinite] * see infinite
源自 拉丁語 onfonotus [無窮大的] *參見 infinite

【引伸】
in.finites.imally
adv.（副詞）