Tracer
There are two main types of tracers in fluids:
- Passive: a tracer which does not effect the flow.
- Active: a tracer which does effect the flow.
Imagine someone thrown into the Atlantic ocean. No matter how hard they swim they do not change the gulfstream flow, so we consider their motion inconsequential. On the other hand potential temperature and potential vorticity are active tracers, and their effects must be taken into account.