If something is tangential, it touches your subject only at the edge — it is connected, but not in the way that matters. You say it about remarks, questions, information, evidence, or a whole discussion: 'a tangential remark', 'the point is tangential to our argument'. Note that pattern: tangential TO something. It is not a rude word, but it is a polite way of saying 'this is off the point', so people use it in meetings and academic writing when they want to move on without embarrassing anyone. It is formal — you would not say it to a friend, where you would say 'that's a bit off-topic' or 'that's beside the point'. Two things to keep straight. First, tangential does not mean unrelated. If something is completely unconnected you say 'irrelevant'; tangential means there IS a link, just a thin one at the edge. Second, people also say a person is tangential, or 'goes off on a tangent', meaning they keep drifting from one side-topic to another — that is about the speaker, not the fact. In maths and physics the word keeps its literal meaning: touching a curve at one point, or acting along the direction of a tangent (tangential velocity, tangential force). The picture behind the everyday meaning comes from there — a line that meets a circle at one single point and then heads off in its own direction. That image is worth holding on to; it explains the word better than any definition.