When people say that "time" is the fourth dimension, what does that mean? The fourth dimension adds a way for the third dimension to change: this is obvious when we say "the third dimension is space without time". But the entropy-driven "arrow of time" that people associate with this concept is obviously not spatial, because it behaves in ways that are different from the first three dimensions. This is why some people prefer to say that the fourth dimension is a "temporal" dimension, while the first three are spatial.

But the more we learn about "space-time" and general relativity, the more we realize that time is not just an arrow. The fourth dimension stretches, it bends, and quantum entanglement shows that it's possible for particles to make instantaneous connections within it, even for there to be causality in time's reverse direction! And as mind-blowing as this may be to fathom, the accepted definition for anti-matter is that it's matter which is moving "backwards in time".