Definition:Infinitesimal

Definition
An infinitesimal is a mathematical object $\delta$ resembling a (real) number such that:
 * $(1): \quad \delta > 0$
 * $(2): \quad \forall x \in \R_{>0}: \delta < x$

That is, an infinitesimal is a (strictly) positive real number which is smaller than every other (strictly) positive real number.