The usual physics terminology for motion that repeats itself over and over is periodic motion, and the time required for one repetition is called the period, often expressed as the letter T. (The symbol P is not used because of the possible confusion with momentum.)

One complete repetition of the motion is called a cycle. The frequency is defined as the number of cycles per unit time. Frequency is usually denoted by a Latin letter for by a Greek letter ν (nu). Note that period and frequency are reciprocals of each other.

\( f=\frac{1}{T} \)

For example, if a newborn baby’s heart beats at a frequency of 120 times a minute, its period (the interval between beats) is half a second. If you calibrate your intuition so that you expect large frequencies to be paired with short periods, and vice versa.