The Collaborative International Dictionary of English v.0.48:

Flexion \Flex"ion\, n. [L. flexio: cf. F. flexion.]
   1. The act of flexing or bending; a turning.
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   2. A bending; a part bent; a fold. --Bacon.
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   3. (Gram.) Syntactical change of form of words, as by
      declension or conjugation; inflection.
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            Express the syntactical relations by flexion. --Sir
                                                  W. Hamilton.
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   4. (Physiol.) The bending of a limb or joint; that motion of
      a joint which gives the distal member a continually
      decreasing angle with the axis of the proximal part; --
      distinguished from extension.
      [1913 Webster]

WordNet (r) 3.0 (2006):

flexion
    n 1: the state of being flexed (as of a joint) [syn: flexure,
         flection, flexion]
    2: deviation from a straight or normal course [syn:
       inflection, flection, flexion]
    3: act of bending a joint; especially a joint between the bones
       of a limb so that the angle between them is decreased [syn:
       flexion, flexure] [ant: extension]