The additive decomposition of a time series can be written as
$$ Y_t = T_t + C_t + S_t + I_t $$
where $T_t$ is the trend component, $C_t$ is the cyclic component, $S_t$ is the seasonal component, and $I_t$ is irregular noise. (Wikipedia)
The seasonal component is a function with a regular period, but the cyclic component has a possibly very irregular period. I've looked up the terms almost-periodic function, aperiodic function, and quasiperiodic function, but none seem to accurately describe such a function.
What is an accurate term for describing cyclic functions without a period?
Like the 11-year sunspot solar cycle? Maybe stockastic : with a random probability distribution or pattern that may be analyzed statistically but may not be predicted precisely.
But it would be understood that you are in this situation of you use the word cyclic.