Physicist: All philosophy aside, the best answer is due to Einstein, who said (after lots of thought) that “Time is what clocks measure”.
The term “spacetime” gets bandied about a lot. And you may have been lead to (correctly) suspect that time and space are closely related, and even a little interchangeable. Time is still not exactly like another spacial dimension (like the usual 3). Which is why physicists will tend to say that we live in “3+1 dimensions”, for 3 spacial dimensions, and 1 time dimension. If someone says that time is “the 4th dimension”, keep in mind that there’s an asterisk attached.
If you’re trying to measure a distance between two events and in 1 dimension you’ll find that , which is just another way of writing “the absolute value of “. In 2 dimensions , which is the Pythagorean theorem. And in 3 dimensions . Notice a pattern? The kick-ass thing about each of these equations is that no matter how you shift or spin around the coordinates, D will always stay the same, which is exactly what you’d expect. If you measure a yard stick at any angle, anywhere in the world, you’ll always measure the same length. “D” is the only equation that makes that happen.
In 3+1 dimensions (the standard spacetime) . Here the x’s, y’s, and z’s are the coordinates in space of event 1 and event 2, the t’s are the time coordinates of those events, and C is the speed of light. “L” is the measure of distance in spacetime, but to keep everything straight it’s called the “spacetime interval”. You may have noticed that sometimes when you use this equation that you’ll be taking the square root of a negative number. Don’t worry about it. The power of the Interval, L, is that, like the D above, L remains unchanged by changing location, rotating, and even movement (holy crap), and it is the only equation that does all that. So hidden away in all that wordiness is the big difference between space and time:
The time term in L has a minus sign. As far as physics goes, that’s about it. All of the horror and weirdness of special relativity (all the time dilation, length contraction, ruined simultaneity, all of it) can be tracked back to that minus sign. If you wanted to, you could define time to be the direction in spacetime that has a minus in front of it in L. I don’t think many people do that (not many grounded people at least), but they could if they really wanted.