The
exponential probability distribution function with parameter $\theta > 0$ is the
D4367: Unsigned real function
\begin{equation}
\mathbb{R} \to [0, 1], \quad
x \mapsto
\begin{cases}
1 - e^{- \theta x}, \quad & x \in [0, \infty) \\
0, \quad & x \in (-\infty, 0)
\end{cases}
\end{equation}