# Spacetime Reference Libraries

Spacetime -- In special relativity and general relativity, time and three-dimensional space are treated together as a single four-dimensional manifold called spacetime (alternatively, space-time; see below). A point in spacetime may be referred to as an event. Each event has four coordinates (t, x, y, z). Just as the x, y, z coordinates of a point depend on the axes one is using, so...

Space -- The definition of space in physics is contentious. Various concepts used to try to define space have included: -- the structure defined by the set of "spatial relationships" between objects -- a manifold defined by a coordinate system where an object can be located. -- the entity that stops all objects in the universe from touching one another In classical physics,...

- The navel or umbilicus.
- In Greek archaeology: A central boss, as on a shield, a bowl, etc.
- A sacred stone in the temple of Apollo at Delphi, believed by the Greeks to mark the 'navel' or exact center-point of the earth.