di·gres·sion /-ˈgrɛʃən/
  離題,扯到枝節上,脫軌
  Di·gres·sion n.
  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.
     The digressions I can not excuse otherwise, than by the confidence that no man will read them.   --Sir W. Temple.
  2. A turning aside from the right path; transgression; offense. [R.]
  Then my digression is so vile, so base,
  That it will live engraven in my face.   --Shak.
  3. Anat. The elongation, or angular distance from the sun; -- said chiefly of the inferior planets. [R.]
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  digression
       n 1: a message that departs from the main subject [syn: 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, deflection, deflexion, divagation]
       3: wandering from the main path of a journey [syn: excursion]