Merriam-Webster Collegiate Dictionary - frequency

 noun  (plural -cies)  Date: 1600  1. the fact or condition of occurring frequently  2.  a. the number of times that a periodic function repeats the same sequence of values during a unit variation of the independent variable  b. the number, proportion, or percentage of items in a particular category in a set of data  3. the number of repetitions of a periodic process in a unit of time: as  a. the number of complete alternations per second of an alternating current  b. the number of complete oscillations per second of energy (as sound or electromagnetic radiation) in the form of waves