digression

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act of digressing.
• noun An instance of digressing, especially a written or spoken passage that has no bearing on the main subject.

from The Century Dictionary.

• noun The act of digressing; deviation from a regular or appointed course; especially, a departure from the main subject under consideration; an excursion of speech or writing.
• noun Deviation from the path of virtue; transgression.
• noun In astronomy, the angular distance in the ecliptic of the inferior planets Mercury and Venus from the sun.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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 rare A turning aside from the right path; transgression; offense.
• noun (Anat.), rare The elongation, or angular distance from the sun; -- said chiefly of the inferior planets.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A departure from the subject, course, or idea at hand; an exploration of a different or unrelated concern.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a turning aside (of your course or attention or concern)
• noun a message that departs from the main subject
• noun wandering from the main path of a journey