Spacetime
Definition
The four-dimensional fabric combining the three dimensions of space with time as a fourth dimension. Everything that happens — every event — has a location in spacetime.
Unit: meters (m) for spacetime intervals
Technical definition
The four-dimensional continuum (three spatial dimensions plus time) in which physical events are located. In special relativity it has a flat Minkowski geometry; in general relativity, mass and energy curve spacetime, and that curvature is what we experience as gravity.
Example
Your phone's GPS must account for spacetime curvature: clocks on satellites tick faster than ground clocks partly because Earth's mass curves spacetime less strongly at orbital altitude — a direct, everyday consequence of general relativity.
Spacetime is not just 'space plus time written together.' The key insight is that space and time mix: different observers disagree on which events are simultaneous, and motion through space affects the passage of time. Treating time as fully separate from space breaks relativity.