Aperiodic
Elementary
We say that something occurs aperiodically if the length of time between each time it happens is not always the same.
High school
A function, signal, or sequence of events is aperiodic if it is not periodic.
Undergraduate (real-time systems)
A sequence of times $t_1$, $t_2$, $t_3$, … is called aperiodic if there does not exist a constant $P > 0$ such that $t_{i + 1} - t_{i} = P$ for each $i = 1, 2, 3, \dots$.