a union of space and time originally emerging from special relativity. Can be viewed as the "fabric" out of which the universe is fashioned; it constitutes the dynamical arena within which the events of the universe take place.
Our local, solid, 3-D living environment (also referred to as the spacetime continuum). The matter/energy building material of our local 3-D universe. A particle evaporation of n-space.
We are used to thinking of our world as having 3 space dimensions and 1 time dimension. From the theory of special relativity we know that space and time are intimately linked. Therefore we combine them into a 4-dimensional spacetime (or 3+1 dimensional spacetime to be precise). Then the trajectory of a moving particle in this spacetime is an invariant object called a 'worldline'. How we measure the space and time dimensions amounts to a preferred set of coordinate axes which depends on our frame of reference. We generalize to higher dimensional spacetimes simply by adding more space dimensions (e.g. 10-dimensional spacetime has 9 space dimensions and 1 time dimension).
the four-dimensional combination of space (three dimensions) and time (the fourth dimension). As a consequence of Special Relativity, time and space are not independent of each other and are relative to the motion of an observer.