Technically, a poolish uses equal weights of flour and water and commercial yeast. However, you can also use equal weights of flour and water with wild yeast and end up with a starter that has the same consistency (same hydration value) as a poolish made with commercial yeast. Some people refer to the natural yeast version as a natural preferment or a natural poolish to distinguish that version from a poolish made with commercial yeast. However, the fact that the hydration values of a natural poolish and a commercial one are the same does not mean that they are interchangeable in recipes or used in the same way in recipes. Moreover, sourdough starters often do not use equal weights of flour and water and, as such, are not a poolish by any definition.