To go off on a tangent isn’t quite the same as the illustrations of Adult Attention Deficit Disorder on e-cards: I was just sitting there and suddenly…oh, look bacon! But to do so can make it difficult to get the train of though back on the track, as it were. It is an ADJECTIVE.
Tangential explained: “Touching lightly; incidental.
As far as Katherine was concerned, everything else was tangential to her own plans.
Someone who starts talking about one thing and gets sidetracked has gone off on a tangent. (In geometry, a tangent is a straight line that touches a curve at a single point.) The new subject is tangential to the first subject–it ‘touches’ it lightly and moves off in a different direction. A few of the people we meet truly enter out lives, but most acquaintances remain only tangential.”
–Merriam-Webster’s Vocabulary Builder, 1998