Smoke is measured in Phenol Parts Per Million. This is a way of gauging just how smoky your dram is.
A phenol is a type of carbolic acid found in smoke tar and as such is present in peated and smoky Whiskies.
This is measured in Parts Per Million, or PPM, which is simply a way of counting the Phenols in the Whisky to be able to say how smoky it is.