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.
      [1913 Webster]

   2. A turning aside from the right path; transgression;
      offense. [R.]
      [1913 Webster]

            Then my digression is so vile, so base,
            That it will live engraven in my face. --Shak.
      [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "digression":
   aberrancy, aberration, ambages, apostrophe, aside, bend, bias,
   branching off, bypass, bypath, byway, circling, circuit,
   circuition, circuitousness, circuitry, circularity, circulation,
   circumambience, circumambiency, circumambulation, circumbendibus,
   circumflexion, circumlocution, circummigration, circumnavigation,
   corner, crook, crosswiseness, curve, declination, deflection,
   deflexure, departure, detour, deviance, deviancy, deviating,
   deviation, deviousness, diagonality, digressing, discursion,
   divagation, divarication, divergence, diversion, dogleg, double,
   drift, drifting, episode, errantry, excurse, excursion, excursus,
   exorbitation, gyre, gyring, hairpin, incident, indirection,
   indirectness, meandering, nonconformity, obiter dictum,
   obliqueness, obliquity, orbit, orbiting, parenthesis, pererration,
   rambling, roundabout, roundabout way, roundaboutness, rounding,
   sheer, shift, shifting, shifting course, shifting path, side path,
   side road, sidetrack, skew, skewness, slant, spiral, spiraling,
   squint, straying, sweep, swerve, swerving, swinging, tack,
   transverseness, turn, turning, twist, vagary, variation, veer,
   wandering, warp, wheeling, yaw, zigzag