a step in walking or running | the outer boundary of an artifact or a material layer constituting or resembling such a boundary |
structural member consisting of the horizontal part of a stair or step | a device that provides reactive force when in motion relative to the surrounding air; can lift or control a plane in flight |
the part (as of a wheel or shoe) that makes contact with the ground | a superficial aspect as opposed to the real nature of something |
the grooved surface of a pneumatic tire | information that has become public |
| the extended two-dimensional outer boundary of a three-dimensional object |
| the outermost level of the land or sea |