What is "grade separation"

n. a crossing that uses an underpass or overpass

Grade separation is the name given to a method of aligning a junction of two or more surface transport axes at different heights ( grades) so that they will not disrupt the traffic flow on other transit routes when they cross each other. The composition of such transport axes does not have to be uniform; it can consist of a mixture of roads, footpaths, railways, canals, or airport runways. Bridges (or overpasses or flyovers), tunnels (or underpasses), or a combination of both can be built at a junction to achieve the needed grade separation.

In North America, a grade-separated junction may be referred to as a grade separation or as an interchange – in contrast with an intersection, at-grade, a diamond crossing or a Level crossing, which are not grade-separated.