Na-adjective (keiyodoshi), Noun
thin (air); weak (liquid); dilute; sparse (population); rarefied
高く登るにつれて、空気は希薄になる。As you go up higher, the air becomes thinner. lacking (enthusiasm, consideration, substance, etc.); insubstantial; deficient; tenuous (link)
Also written 稀薄【きはく】