Osculate

1. trans. To kiss, salute with contact of the lips; intr. to kiss each other. rare.

2. trans. To bring into close contact or union.

3. intr. To come into close contact or union; to have close contact with each other, to come together. In Nat. Hist. To have contact through an intermediate species or genus (cf. osculant).

4. Math. trans. To have contact of a higher order with, esp. the highest contact possible for two loci; to have three or more coincident points in common with; intr. (for refl.) to osculate each other: as two curves, two surfaces, or a surface and a curve.

Hence "osculating ppl. a., usually in sense 4, as osculating circle, curve, plane, sphere.