nonsequitur
noun
Definitions
Any abrupt and inexplicable transition or occurrence.
Example: Having a costumed superhero abduct the vicar was an utter non sequitur in the novel.
Any invalid argument in which the conclusion cannot be logically deduced from the premises; a logical fallacy.
A statement that does not logically follow a statement that came before it.
(humor) A kind of pun that uses a change of word, subject, or meaning to make a joke of the listener’s expectation.