(optics) A surface to which rays reflected or refracted by another surface are tangents.

(idiomatic) To barely begin; to see or do only a fraction of what is possible.

capable of lowering the surface tension of a liquid

(physics) the effect on the surface of a liquid that makes it behave as a stretched elastic membrane; it is caused by unbalanced intermolecular forces / (physics) a measure of this effect

(of a missile) fired from the ground, or from a ship, at an aircraft in flight, or at an incoming missile

(of a missile) fired from an aircraft at a target on land or water

Overland flow of excess water (with or without accumulated contaminants) that cannot be absorbed by the ground as infiltration.

(geometry) A surface (in more than two dimensions, a plane is trivially developable) that can be flattened into a plane without distortion.

A numerical description of the flatness or roughness of a surface, especially an optical or machined surface.

(mathematics) A one-dimensional complex manifold; a generalization of the complex plane.