digression

Short definition from Wiktionary

Noun

1. 1
   An aside, an act of straying from the main subject in speech or writing.
2. 2
   The act of straying from the main subject in speech or writing, (rhetoric) particularly for rhetorical effect.
3. 3
   A deviancy, a sin or error, an act of straying from the path of righteousness or a general rule.
4. 4
   A deviation, an act of straying from a path.
5. 5
   An elongation, a deflection or deviation from a mean position or expected path.