x1 (linear manifold, vector, etc.) is/lies tangent to x2 (object, surface, curve, manifold, etc.) at point/locus x3, by standard/definition/in system x4.
No necessary implication of co-directionality. Prone to mathematical/geometric generalization. It can be used metaphorically as in English for a line of discussion (x1) which only barely relates to or touches the general topic or discussion (x2), the latter acting as the primary/main/current/relevant/focal topic or discussion, due to or at a connecting point/detail/particular subtopic/idea (x3), with x1 being an irrelevant or unimportant or peripherally branching line of thought. See also: panra, kukru; tanjo, zo'a, tsekane; ta'o.