This is an approach to morphology-phonology interaction. Whenever an affix is added (for instance to a stem), a phonological rule applies. E.g. the English word cyclicity itself consists of [cyclic] + [ity]. When -ity is attached, the final [k] changes into [s]: we call this cyclic rule application, or we can refer to -ity as a "cyclic" affix. (Note that the word cyclic itself can also be analysed as morphologically complex: [cycle] + [ic])