In the theory of relativity, spacetime is a mathematical concept that combines space and time into a single framework for understanding the physical universe. According to this theory, the three dimensions of space and the one dimension of time are not separate and distinct, but are instead closely intertwined and cannot be understood independently of one another. In other words, spacetime is a four-dimensional structure that describes the way that objects move and interact in the universe. One way to think about spacetime is to imagine it as a fabric that is stretched out in all directions. The positions and movements of objects in the universe can be thought of as points or lines on this fabric. The fabric itself is not made up of matter, but rather it is a framework that provides the structure for the matter and energy that exist within it.