Case of a vibrational motion is considered when the process is close to periodical one but is not periodical because the frequency of the process is not constant. A model is proposed for description of the processes. The model uses a subclass of recursive functions. Two definitions of the functions are given. The first definition is based on a periodic pattern and the second one - on the theoretical-set language. The equivalence of introduced definitions is proved. It is noted that the model on the base of recursive functions permits to investigate the specific features of a vibrational motion.