(physics) an imaginary surface joining all points in space that are reached at the same instant by a wave propagating through a medium | the outer boundary of an artifact or a material layer constituting or resembling such a boundary |
| a device that provides reactive force when in motion relative to the surrounding air; can lift or control a plane in flight |
| a superficial aspect as opposed to the real nature of something |
| information that has become public |
| the extended two-dimensional outer boundary of a three-dimensional object |
| the outermost level of the land or sea |