Merging (merged range)
The term merging refers to the process of adjusting your range in a way that prevents it from appearing polarised. This means that the range of the player has to contain more than only two opposing hand strengths. By merging your range, you make it impossible for your opponents to put you on an accurate range in the long term. This in turn will inevitably force them into mistakes.
If a player only re-raises sets on this dry flop, his range is polarised and unbalanced. If he occasionally also re-raises air in addition to sets, his range remains polarised, but is balanced between very strong hands (sets) and air. If the player takes it one step further and also re-raises sets as well as air and top pair hands, his range is unpolarised and balanced (= merged range).