Touching at exactly one point, which the test always encodes as Δ = 0 on the combined equation. Every "tangent to," "touches at exactly one point," "intersects exactly once," or "has a double root" item is Δ=0; "does not intersect" is Δ<0; "intersects twice" is Δ>0. Recognizing the vocabulary that invokes the discriminant is most of the skill.