noun (n)

• a message that departs from the main subject(noun.communication)

  Synonym:
  aside, divagation, excursus, parenthesis
  

  source: wordnet30
• a turning aside (of your course or attention or concern)(noun.act)

  Example:
  A digression into irrelevant details.
  

  Synonym:
  deflection, deflexion, deviation, divagation, diversion
  

  source: wordnet30
• wandering from the main path of a journey(noun.act)

  Synonym:
  excursion
  

  source: wordnet30
• 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.(noun)

  source: webster1913