noun

• (generally uncountable) The act of straying from the main subject in speech or writing, (rhetoric) particularly for rhetorical effect.
  "make digression... by way of digression..."
• (obsolete) A deviancy, a sin or error, an act of straying from the path of righteousness or a general rule.
  "Nature... More stronger hadde her operacyon Than she hath nowe in her dygressyon."
• (now rare) A deviation, an act of straying from a path.
  "By this little digression into Gascony, the Duke had an opportunity... to re-inforce himself with some particular Servants of his."
• (astronomy, physics) An elongation, a deflection or deviation from a mean position or expected path.
  "This digression [of the Sun] is not equall, but neare the Æquinoxiall intersections, it is right and greater, near the Solstices, more oblique and lesser."