What is meant by returns to a factor? 

Return to a factor means, 'Change' in total physical product when an additional unit of a variable factor is employed with fixed factors'. Alternatively 'when only one factor is increased keeping all other factors fixed, the resultant increase in output is called returns to a factor'. It is also known as 'law of return to a factor' although modern economists call it as 'law of variable proportion' because with increase in variable input, the proportion between variable factors and fixed factors gets changed.