The Collaborative International Dictionary of English v.0.48:

Quaternion \Qua*ter"ni*on\, v. t.
   To divide into quaternions, files, or companies. --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Quaternion \Qua*ter"ni*on\, n. [L. quaternio, fr. quaterni four
   each. See Quaternary.]
   1. The number four. [Poetic]
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   2. A set of four parts, things, or person; four things taken
      collectively; a group of four words, phrases,
      circumstances, facts, or the like.
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            Delivered him to four quaternions of soldiers.
                                                  --Acts xii. 4.
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            Ye elements, the eldest birth
            Of Nature's womb, that in quaternion run. --Milton.
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            The triads and quaternions with which he loaded his
            sentences.                            -- Sir W.
                                                  Scott.
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   3. A word of four syllables; a quadrisyllable.
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   4. (Math.) The quotient of two vectors, or of two directed
      right lines in space, considered as depending on four
      geometrical elements, and as expressible by an algebraic
      symbol of quadrinomial form.
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   Note: The science or calculus of quaternions is a new
         mathematical method, in which the conception of a
         quaternion is unfolded and symbolically expressed, and
         is applied to various classes of algebraical,
         geometrical, and physical questions, so as to discover
         theorems, and to arrive at the solution of problems.
         --Sir W. R. Hamilton.
         [1913 Webster]

WordNet (r) 3.0 (2006):

quaternion
    n 1: the cardinal number that is the sum of three and one [syn:
         four, 4, IV, tetrad, quatern, quaternion,
         quaternary, quaternity, quartet, quadruplet,
         foursome, Little Joe]

Easton's 1897 Bible Dictionary:

Quaternion
   a band of four soldiers. Peter was committed by Herod to the
   custody of four quaternions, i.e., one quaternion for each watch
   of the night (Acts 12:4). Thus every precaution was taken
   against his escape from prison. Two of each quaternion were in
   turn stationed at the door (12:6), and to two the apostle was
   chained according to Roman custom.