"Space and duration are one."
Edgar Allan Poe, in Eureka, published 1848
Any event is a point (or region) in spacetime, that can be defined by it's place and it's time. The place and time can be described by a four-number coordinate system, often expressed relative to the observer (and thus the observer's Frame of Reference). This coordinate system, because of the incorporation of the time element, incorporates some interesting geometry. For example, the orbit of a planet can be described as an ellipse in space, but is helical in spacetime.
The first major breakthrough in regards to spacetime was achieved by Albert Einstein, and it plays a cornerstone role in his theories of Relativity, where he proposes that spacetime is curved. Certain aspects of the dimensional notion of time were earlier proposed by other creative thinkers, such as William Rowan Hamilton,Bernhard Riemann, Edgar Allen Poe, and H.G. Wells.
Prior to Einstein, most of science considered space to have three dimensions, and time to be something different that had only a single dimension. The equations behind Relativity have shown that "3+1" perspective to be flawed.
- Closed Timelike Curve