The Collaborative International Dictionary of English v.0.48:

Frequency \Fre"quen*cy\, n.; pl. Frequencies. [L. frequentia
   numerous attendance, multitude: cf. F. fr['e]quence. See
   Frequent.]
   1. The condition of returning frequently; occurrence often
      repeated; common occurence; as, the frequency of crimes;
      the frequency of miracles.
      [1913 Webster]

            The reasons that moved her to remove were, because
            Rome was a place of riot and luxury, her soul being
            almost stifled with, the frequencies of ladies'
            visits.                               --Fuller.
      [1913 Webster]

   2. A crowd; a throng. [Obs.] --B. Jonson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

frequency
    n 1: 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" [syn:
         frequency, frequence, oftenness]
    2: the ratio of the number of observations in a statistical
       category to the total number of observations [syn:
       frequency, relative frequency]
    3: the number of observations in a given statistical category
       [syn: frequency, absolute frequency]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "frequency":
   AF, CPS, EHF, HF, Hz, MF, RF, SHF, UHF, VHF, VLF, amplitude,
   antinode, audio frequency, carrier frequency, crest, cycles,
   de Broglie wave, diffraction, electromagnetic radiation,
   electromagnetic wave, extremely high frequency, fluctuation,
   frequency band, frequency spectrum, fundamental, fundamental tone,
   guided wave, harmonic, harmonic motion, hertz, high frequency,
   in phase, interference, intermediate frequency, intonation,
   kilocycles, kilohertz, libration, light, longitudinal wave,
   low frequency, lower frequencies, mechanical wave,
   medium frequency, megacycles, megahertz, monotone, monotony, node,
   nutation, oscillation, out of phase, overtone, partial,
   partial tone, pendulation, period, periodic wave, periodicity,
   pitch, radio frequency, radio wave, ray, reinforcement, resonance,
   resonance frequency, seismic wave, shock wave, sound wave,
   spark frequency, spectrum, superhigh frequency, surface wave,
   tidal wave, tone, tonelessness, transverse wave, trough,
   ultrahigh frequency, upper frequencies, vacillation,
   very high frequency, very low frequency, vibrancy, vibratility,
   vibration, wave, wave equation, wave motion, wave number,
   wavelength, wavering