(architecture) An inverted arch (as in a sewer). *
The base of a tunnel on which the road or railway may be laid and used when construction is through unstable ground. It may be flat or form a continuous curve with the tunnel arch. [1]
(civil engineering) The lowest point inside a pipe at a certain point.
(civil engineering) An elevation of a pipe at a certain point along the pipe.
A skateboarding trick where the skater grabs the board and plants a hand on the coping so as to balance upside-down on the lip of a ramp.