noun

a temporary departure from the main subject in speech or writing.

let’s return to the main topic after that brief digression

deviation,detour,diversion,departure,excursus,aside,incidental remark,footnote,parenthesis,deviation from the subject,straying from the topic,straying from the point,going off at a tangent,getting sidetracked,losing one’s thread,divergence,straying,drifting,rambling,wandering,meandering,maundering,obiter dictum,excursion,apostrophe,divagation