So what did I think timing and spacing are? I thought timing should be everything that's related to... well, timing... and spacing, guess what, yeah, all that's related to spacing, or to an object's position in space, or along its path of action... It made sense to me to have a simple, solid explanation such as this, and I didn't really understand what these concepts were really used for, by other animators.
Not that it matters much to understand timing and spacing theoretically.... I think... if you can apply them well enough in practice :D
But anyway, here's the definition now, hopefully correct :D
Timing and spacing are abstract concepts created by animators to be used as animation tools.
Timing is easier to explain, it represents how much time passes between two keys.
Spacing means what happens between the two keys: if the movement is even, if it accelerates, or if it slows down. Basically, if there is more space between poses, visually, the object will move faster. And the other way around. So spacing relates to time more than space, and is actually a timing tool, because the difference in spacial positioning creates a difference in speed.