Osculation means kissing. If you tease your friends with a rhyme that describes them "sitting in a tree/k-i-s-s-i-n-g," you're joking about osculation.
Most wedding ceremonies traditionally end with osculation, and osculation is also part of a typical greeting in many parts of Europe. It's a fancy way of saying "kiss," and if you think it sounds vaguely mathematical, you're right. Osculation is also a term in geometry that describes the place where two curves or surfaces come into contact, or where their common tangent exists. It's almost as if the two curved lines are kissing.