I suppose a better definition would be that sin taxes are meant to cut down on personal use, while pigovian are meant to offset external factors which the use of the product/service causes. Of course, each of these taxes are going to end up cutting down use, as it would lower demand due to a higher cost, but the idea is that a pigovian tax is successful even if the use doesn't go down significantly, as the tax proceeds would be reinvested in offsets from the negative activity. A sin tax could do this, but it doesn't always do this.