タイツとストッキングの違いはデニール(線密度、繊度、糸・繊維の太さの単位)で、30未満がストッキング、30以上がタイツなんだって

The difference between tights and pantyhose is their denier (a unit of linear density, fineness or thread/fiber thickness): leggings under 30 D are pantyhose, and those at 30+ D are tights.