The Collaborative International Dictionary of English v.0.48:

Straighten \Straight"en\, v. t. [imp. & p. p. Straighted; p.
   pr. & vb. n. Straighting.]
   1. To make straight; to reduce from a crooked to a straight
      form.
      [1913 Webster]

   2. To make right or correct; to reduce to order; as, to
      straighten one's affairs; to straighten an account.
      [1913 Webster]

   To straighten one's face, to cease laughing or smiling,
      etc., and compose one's features.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Straighten \Straight"en\, v. t.
   A variant of Straiten. [Obs. or R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

straighten
    v 1: straighten up or out; make straight [syn: straighten,
         unbend] [ant: bend, flex]
    2: make straight [syn: straighten, straighten out]
    3: get up from a sitting or slouching position; "The students
       straightened when the teacher entered"
    4: put (things or places) in order; "Tidy up your room!" [syn:
       tidy, tidy up, clean up, neaten, straighten,
       straighten out, square away]
    5: straighten by unrolling; "roll out the big map" [syn: roll
       out, straighten]
    6: make straight or straighter; "Straighten this post";
       "straighten hair"