2.NounA liquid substance used to thin the consistency of another liquid.
"Turpentine or mineral spirits can be used as a thinner for oil-based paints."
3.NounSomething that thins or thins out (another thing).
"Pomegranate juice and exercise are great natural blood thinners.; The new type of mechanical thinner allowed thinning of peach blossoms to be completed earlier.; A solid mane thinner is indispensable in finishing the looks of your horse."