The Collaborative International Dictionary of English v.0.48:

Digression \Di*gres"sion\, n. [L. digressio: cf. F. digression.]
   1. The act of digressing or deviating, esp. from the main
      subject of a discourse; hence, a part of a discourse
      deviating from its main design or subject.
      [1913 Webster]

            The digressions I can not excuse otherwise, than by
            the confidence that no man will read them. --Sir W.
                                                  Temple.
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   2. A turning aside from the right path; transgression;
      offense. [R.]
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            Then my digression is so vile, so base,
            That it will live engraven in my face. --Shak.
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   3. (Anat.) The elongation, or angular distance from the sun;
      -- said chiefly of the inferior planets. [R.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

digression
    n 1: a message that departs from the main subject [syn:
         digression, aside, excursus, divagation,
         parenthesis]
    2: a turning aside (of your course or attention or concern); "a
       diversion from the main highway"; "a digression into
       irrelevant details"; "a deflection from his goal" [syn:
       diversion, deviation, digression, deflection,
       deflexion, divagation]
    3: wandering from the main path of a journey [syn: digression,
       excursion]