What is the difference between a Variac, variable autotransformer and Dimmerstat?

An Autotransformer is a transformer with only one winding. See more. A variable autotransformer is an autotransformer with a sliding scale which can be used to adjust the transformer ratio. "Variac" and "Dimmerstat" are trade names of variable autotransformers. Hence, they are the same.