Frequency measures how often things repeat over time. City buses often reach stops at a frequency of every 15 minutes, unless it's snowing or raining really hard. In that case, the frequency will slow.

You probably know the word frequent, a synonym for often. So, it is tempting to think frequency describes something that happens often. However, this isn't true: frequency describes any rate of time at which something repeats. For example, from Earth, Halley's Comet is visible at a frequency of 76 years — it's not frequent, but it is on schedule.

Definitions of frequency
  1. noun
    the number of occurrences within a given time period
    “the frequency of modulation was 40 cycles per second”
    “the frequency of his seizures increased as he grew older”
    synonyms: frequence, oftenness
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    audio, audio frequency
    an audible acoustic wave frequency
    radio frequency
    an electromagnetic wave frequency between audio and infrared
    infrared, infrared frequency
    the infrared region of the electromagnetic spectrum; electromagnetic wave frequencies below the visible range
    wave number
    the reciprocal of the wavelength of a wave
    the frequency with which a person is present
    count per minute, counts/minute
    frequency per minute
    sampling frequency
    (telecommunication) the frequency of sampling a continuously varying signal
    the frequency assigned to a broadcasting station
    ELF, extremely low frequency
    below 3 kilohertz
    VLF, very low frequency
    3 to 30 kilohertz
    LF, low frequency
    30 to 300 kilohertz
    MF, medium frequency
    300 to 3000 kilohertz
    HF, high frequency
    3 to 30 megahertz
    VHF, very high frequency
    30 to 300 megahertz
    UHF, ultrahigh frequency
    300 to 3000 megahertz
    SHF, superhigh frequency
    3 to 30 gigahertz
    EHF, extremely high frequency
    30 to 300 gigahertz
    rydberg, rydberg constant, rydberg unit
    a wave number characteristic of the wave spectrum of each element
    Nyquist frequency
    (telecommunication) twice the maximum frequency occurring in the transmitted signal
    type of:
    a magnitude or frequency relative to a time unit
  2. noun
    the number of observations in a given statistical category
    synonyms: absolute frequency
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    type of:
    cardinal, cardinal number
    the number of elements in a mathematical set; denotes a quantity but not the order
  3. noun
    the ratio of the number of observations in a statistical category to the total number of observations
    synonyms: relative frequency
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    incidence, relative incidence
    the relative frequency of occurrence of something
    the relative incidence of a particular disease
    type of:
    the relative magnitudes of two quantities (usually expressed as a quotient)
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