Definitions for **"Infinitesimal"**

Infinitely or indefinitely small; less than any assignable quantity or value; very small.

An infinitely small quantity; that which is less than any assignable quantity.

Infinitesimal is a variable whose limit is zero. Development by Abraham Robinson (1960) of the Nonstandard Analysis conferred new significance on infinitesimals and brought them closer to the vision of Leibniz (1646-1716) who introduced the dy/dx notation for the derivative and perceived infinitesimals more like small but constant quantities.