If the 4th dimension is time, what is the 5th dimension? Answer like a physicist.
The fifth dimension is probably the gravitational field. In general relativity it can be described by the curvature of spacetime caused by matter or energy. This curvature has a direct effect on how objects move and interact with each other, so including it in our description of reality would make sense. However, we don't have any evidence that such a fifth dimension exists, or if it does whether it's related to time. So for now I'll stick to four dimensions.