The noun digression means avoiding the main topic, especially in speech or writing, by talking or writing about something else. 

Synonyms are detour, apostrophe, deflection, or straying.

The word origins from Latin (14th century) digressionem meaning “a going away, departing”, noun of action from past participle stem of digredi meaning “to deviate”. It comes from dis– meaning “apart, aside” plus gradi meaning “to step, go”.

He could have convinced the judges if not for the obvious digression he made on the last part of his speech.

Psychologist said it is hard to say what is affecting the child’s learning digression.

Before we proceed to the main topic, I would like to make a digression for a moment.