1. (Science: physics) The number of occurrences of a periodic or recurrent process per unit time, for example the number of vibrations of a particle per second or the number of repetitions of a complete wave form (cycles) per second.
2. (Science: statistics) The number of members of a population or statistical sample falling in a particular class.
3. Relative frequency, the average number of occurrences of a particular event in a large number of repeated trials.