Tangentially is the adverb of tangential: it means only at the edge, in a side way, not in the way that really counts. In practice you meet it in two shapes. Most often it sits with 'related', 'connected', or 'linked': 'the two cases are only tangentially related', 'he was tangentially involved in the scandal'. Almost always there is an 'only' or 'at most' nearby, because the whole job of the word is to shrink a connection down — to admit it exists while saying it is thin. The second shape is about how someone speaks or writes: 'she mentioned it tangentially' means she brought it up in passing, off to the side of her main point, not head-on. This is a formal word, at home in reports, journalism, and academic writing; in conversation people say 'sort of related' or 'in a roundabout way'. It also has a plain physical meaning in science: moving or acting along a tangent, at right angles to the line from the centre — 'the fragment flew off tangentially'. That literal picture is the source of everything else: a line that meets a circle at one point and then leaves in its own direction.