non sequitur

noun

1. Logic. an inference or a conclusion that does not follow from the premises.
2. a statement containing an illogical conclusion.

noun

1. a statement having little or no relevance to what preceded it
2. logic a conclusion that does not follow from the premises

Latin, literally “it does not follow.” A thought that does not logically follow what has just been said: “We had been discussing plumbing, so her remark about astrology was a real non sequitur.” Non sequitur is Latin for “It does not follow.”