The Collaborative International Dictionary of English v.0.48:

Tetrameter \Te*tram"e*ter\, n. [L. tetrametrus, Gr. ?; te`tra-
   (see Tetra-) + ? a measure: cf. F. t['e]tram[`e]tre.] (GR.
   & Latin Pros.)
   A verse or line consisting of four measures, that is, in
   iambic, trochaic, and anapestic verse, of eight feet; in
   other kinds of verse, of four feet.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Verse \Verse\ (v[~e]rs), n. [OE. vers, AS. fers, L. versus a
   line in writing, and, in poetry, a verse, from vertere,
   versum, to turn, to turn round; akin to E. worth to become:
   cf. F. vers. See Worth to become, and cf. Advertise,
   Averse, Controversy, Convert, Divers, Invert,
   Obverse, Prose, Suzerain, Vortex.]
   1. A line consisting of a certain number of metrical feet
      (see Foot, n., 9) disposed according to metrical rules.
      [1913 Webster]

   Note: Verses are of various kinds, as hexameter,
         pentameter, tetrameter, etc., according to the
         number of feet in each. A verse of twelve syllables is
         called an Alexandrine. Two or more verses form a
         stanza or strophe.
         [1913 Webster]

   2. Metrical arrangement and language; that which is composed
      in metrical form; versification; poetry.
      [1913 Webster]

            Such prompt eloquence
            Flowed from their lips in prose or numerous verse.
                                                  --Milton.
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            Virtue was taught in verse.           --Prior.
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            Verse embalms virtue.                 --Donne.
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   3. A short division of any composition. Specifically:
      [1913 Webster]
      (a) A stanza; a stave; as, a hymn of four verses.
          [1913 Webster]

   Note: Although this use of verse is common, it is
         objectionable, because not always distinguishable from
         the stricter use in the sense of a line.
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      (b) (Script.) One of the short divisions of the chapters
          in the Old and New Testaments.
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   Note: The author of the division of the Old Testament into
         verses is not ascertained. The New Testament was
         divided into verses by Robert Stephens [or Estienne], a
         French printer. This arrangement appeared for the first
         time in an edition printed at Geneva, in 1551.
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      (c) (Mus.) A portion of an anthem to be performed by a
          single voice to each part.
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   4. A piece of poetry. "This verse be thine." --Pope.
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   Blank verse, poetry in which the lines do not end in
      rhymes.

   Heroic verse. See under Heroic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tetrameter
    n 1: a verse line having four metrical feet