\begin{definition}[Definition:Infinitesimal]
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.
\end{definition}