adjective.
Capable of having values approaching zero as a limit.
noun.
An immeasurably or incalculably minute amount or quantity.
noun.
A function or variable continuously approaching zero as a limit.
The Century Dictionary and Cyclopedia
Infinitely or indefinitely small; less than any assignable quantity.
noun.
In mathematics, a fictitious quantity so small that by successive additions to itself no sensible quantity, such as the unit of quantity, could ever be generated.
the GNU version of the Collaborative International Dictionary of English
adjective.
Infinitely or indefinitely small; less than any assignable quantity or value; very small.
adjective.
the different and the integral calculus, when developed according to the method used by Leibnitz, who regarded the increments given to variables as infinitesimal.
noun.
An infinitely small quantity; that which is less than any assignable quantity.