noun, plural fre·quen·cies.

  1. Also fre·quence. the state or fact of being frequent; frequent occurrence: We are alarmed by the frequency of fires in the neighborhood.
  2. rate of occurrence: The doctor has increased the frequency of his visits.
  3. Physics.
    1. the number of periods or regularly occurring events of any given kind in unit of time, usually in one second.
    2. the number of cycles or completed alternations per unit time of a wave or oscillation. Symbol: F; Abbreviation: freq.
  4. Mathematics. the number of times a value recurs in a unit change of the independent variable of a given function.
  5. Statistics. the number of items occurring in a given category.

noun plural -cies

  1. the state of being frequent; frequent occurrence
  2. the number of times that an event occurs within a given period; rate of recurrence
  3. physics the number of times that a periodic function or vibration repeats itself in a specified time, often 1 second. It is usually measured in hertzSymbol: ν, f
  4. statistics
    1. the number of individuals in a class (absolute frequency)
    2. the ratio of this number to the total number of individuals under survey (relative frequency)
  5. ecology
    1. the number of individuals of a species within a given area
    2. the percentage of quadrats that contains individuals of a species

1530s, from French fréquence, from Latin frequentia (see frequent).


1640s, “fact of occurring often,” from Latin frequentia “a crowding, crowd,” from frequentem (see frequent).

Earlier it had been used in a now-obsolete sense of “state of being crowded” (mid-16c.); sense in physics, “rate of recurrence,” especially of a vibration, is from 1831. In radio electronics, frequency modulation (1922, abbreviated F.M.) as a system of broadcasting is distinguished from amplitude modulation (or A.M.).

  1. Physics The rate at which a repeating event occurs, such as the full cycle of a wave. Frequencies are usually measured in hertz. Compare amplitude. See also period.
  2. Mathematics The ratio of the number of occurrences of some event to the number of opportunities for its occurrence.

In physics, the number of crests of a wave that move past a given point in a given unit of time. The most common unit of frequency is the hertz (Hz), corresponding to one crest per second. The frequency of a wave can be calculated by dividing the speed of the wave by the wavelength. Thus, in the electromagnetic spectrum, the wavelengths decrease as the frequencies increase, and vice versa.

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