Quasi-birth–death process
In queueing models, a discipline within the mathematical theory of probability, the quasi-birth–death process describes a generalisation of the birth–death process. As with the birth-death process it moves up and down between levels one at a time, but the time between these transitions can have a more complicated distribution encoded in the blocks.
Source: Wikipedia — Quasi-birth–death process (CC BY-SA 4.0)