Well you could take a look at the posted image at post #3. Potentiometer on the left has three terminals. Staying on the rheostat part of the image (right side), you can see an arrow in the middle of the rheostat. When you change rheostat's resistance, this arrow can travel from the left (R=0) to the right (R=max). Assuming a perfect linear relation between distance and ohmic value, then the arrow in the middle means R/2.
You could construct a rheostat from a potentiometer, by shorting the middle terminal with one of its side terminals.
Finally you could spot another difference between the left and the right part of the image. Potentiometer is used to regulate load's voltage and rheostat is used to regulate load's current.
For potentiometer yes. For rheostat no, there are only two terminals. Maybe the below image could make you understand this better:
As you can see, the third terminal of the potentiometer on the right side is unconnected in this topology, so clearly now we have only two terminals. Potentiometer as a rheostat : DC CIRCUITS