One key thing under the "legal" definitions is that the rainbow trout must be able to reach the Ocean (for west coast). So it is size (varies by state) and in anadromous waters (those where fish can reach the ocean). I am sure that in rivers that flow into the Great Lakes, rainbow trout are refered to as steelhead also.

For Ca:
For purposes of these regulations, a steelhead trout is defined as any rainbow trout greater than 16 inches in length found in anadromous waters.