Light-like
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In physics, the adjective light-like refers to a contour in spacetime in the context of special relativity whose proper length vanishes. A synonym of "light-like" (which is chosen because the worldline of a light ray is an example of a light-like curve) is null (because the proper length equal zero).
This word is used for higher-dimensional manifolds (or surfaces), too, in which case it means that at least one direction at each point is null.