tangent
n. countablen. a sudden change in a conversation or story to a completely different topic. You use this when someone starts talking about something that is not related to the main point.
n. a sudden divergence from a previous course of thought or action. Often used in the phrase 'go off on a tangent' to describe a speaker who becomes distracted by a secondary or unrelated topic.
He went off on a tangent about his cat during the meeting.
The professor started with a lecture on physics but went on a long tangent about his summer vacation.
While the initial argument was sound, the author frequently veered off on a tangent regarding historical anecdotes that did little to support the primary thesis.
Borrowed from Latin tangentem, the accusative of tangēns (“touching”) (in the phrase līnea tangēns (“a touching line”)), the present participle of the verb tangō (“touch”, verb), from Proto-Italic tangō, from Proto-Indo-European teh₂g- (“to touch”). Cognate with Old English þaccian (“to touch lightly, pat, stroke”). More at thack, thwack.
Commonly used with the verb 'go' and the preposition 'on' ('to go off on a tangent').