Thin is the most inclusive of these terms and is interchangeable with any of the others, though not without some loss of precision or of specific connotations. Basically it implies reduction in thickness or in density; in extended use it implies a comparable diminution (as of strength, depth, or intensity).
Attenuate implies thinning as the result of some such process as drawing out, spinning fine, or culturing (as a strain of bacteria) repeatedly or as the effect of conditions (as disease or starvation) which emaciate.
In its frequent extended use attenuate implies the loss of properties that are necessary to a thing’s strength, richness, effectiveness, or vitality, and it often connotes overrefining, oversubtilizing, or overemphasis of an opposing quality.
Extenuate in a somewhat learned use can suggest attrition either by literally emaciating and exhausting (see also PALLIATE ) or by a gradual diminishing of a thing’s importance or significance.
Dilute implies a thinning of what is concentrated by the addition or, in extended use, sometimes by the influence of something that weakens it, neutralizes it, or destroys its vigor or intensity.
Rarefy implies a thinning in density and usually an expansion in volume or a decrease in weight or pressure. The word occurs in extended use chiefly with reference to ideas, emotions, and intellectual powers, sometimes suggesting their spiritualization or refinement and the elimination of all grossness and impurity and sometimes imputing to them a vaporous or tenuous quality.