In the context of thinning something, which word fits?

The term "rarefy" is most closely associated with the concept of thinning, particularly in the context of reducing density or making something less substantial. To rarefy means to make something less dense or to decrease its concentration. This can be applied in various contexts, such as in science where one might rarefy a gas by allowing it to expand, resulting in a thinner composition.

In contrast, while "prune" also suggests the idea of selectively cutting back or reducing, it is specific to plants and refers to removing branches or stems to promote healthy growth. "Sever" implies cutting off or separating completely, often in a more decisive or forceful manner, while "rectify" means to correct or set something right, which does not align with the idea of thinning. Thus, "rarefy" is the word that best encapsulates the action of making something less dense or concentrated.