The Collaborative International Dictionary of English v.0.48:

extenuating \extenuating\ adj.
   serving or tending to reduce the severity of guilt or
   blameworthiness; as, extenuating circumstances.

   Syn: mitigating.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Extenuate \Ex*ten"u*ate\, v. t. [imp. & p. p. Extenuated; p.
   pr. & vb. n. Extenuating.] [L. extenuatus, p. p. of
   extenuare to make thin, loosen, weaken; ex out + tenuare to
   make thin, tenuis thin. See Tenuity.]
   1. To make thin or slender; to draw out so as to lessen the
      thickness.
      [1913 Webster]

            His body behind the head becomes broad, from whence
            it is again extenuated all the way to the tail.
                                                  --Grew.
      [1913 Webster]

   2. To lessen; to palliate; to lessen or weaken the force of;
      to diminish the conception of, as crime, guilt, faults,
      ills, accusations, etc.; -- opposed to aggravate.
      [1913 Webster]

            But fortune there extenuates the crime. --Dryden.
      [1913 Webster]

            Let us extenuate, conceal, adorn the unpleasing
            reality.                              --I. Taylor.
      [1913 Webster]

   3. To lower or degrade; to detract from. [Obs.]
      [1913 Webster]

            Who can extenuate thee?               --Milton.

   Syn: To palliate; to mitigate. See Palliate.
        [1913 Webster]

WordNet (r) 3.0 (2006):

extenuating
    adj 1: partially excusing or justifying; "extenuating
           circumstances"

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "extenuating":
   altering, apologetic, assuasive, bounding, diminishing, excusatory,
   excusing, extenuative, extenuatory, justificatory, justifying,
   lenitive, lessening, limitative, limiting, mitigating, mitigative,
   mitigatory, modificatory, modifying, modulatory, palliative,
   qualificative, qualificatory, qualifying, refuting, rehabilitative,
   restricting, restrictive, softening, tempering, vindicative,
   vindicatory