Various definition of return ratio in a feedback circuit
For a classical control system with negative feedback (negative loop gain) the ratio H=output/input is 

H=Hf/(1+T)   

with Hf=Forward gain and T=-(Loop gain).

1) Hence, T is the product of all transfer functions of the blocks forming the feedback loop - excluding the minus sign at the summing block. Another name of "T" is "return ratio". Hence, the "return ratio" is nothing else than the negative loop gain. 

2.) More than that, the complete denominator F=(1+T) is called "return difference".

Comment: The above terms ("return ratio" and "return difference") are not in common use today. They were defined by Bode in the 40th of the last century. It is not easy to understand or to "visualize" these definitions because they were invented by H.W. Bode based on transfer functions defined as input-to-output ratio. Today, we are using the inverse definition for the transfer function and - as a consequence - the term "loop gain".