Pure ethanol (the stuff that makes you drunk) that has had
impurities deliberately added to it, mainly
methanol. Medical grade methylated spirits are 95 per cent
ethanol, 5 per cent
methanol. Commercial grade methylated spirits also has small concentrations of
naphthalene and
kerosene.
Methylated spirits are sometimes simply known as "metho". The idea behind metho is this: ethanol is a generally useful solvent and handy for lots of things, like fuel, cleaning and so on. However, it is also intoxicating and people who want to get drunk would just buy ethanol and drink it straight or dilute it, probably blowing away their livers in the process. Plus, strictly speaking, since it is arguably an alcoholic beverage, it should be taxed an exhorbitant amount. How do you solve the problem? You add a poison to the ethanol that's hard to separate from it, so people don't drink it, while at the same time not siginificantly changing its useful properties. Hence adding methanol. Methanol is (a) poisonous, (b) an alcohol very similar to ethanol anyway, so likely to have the same properties. It's no longer an alcoholic beverage any more, so now you don't have to pay taxes. But it's still useful as a solvent, cleaning agent, fuel etc.
This has sort of backfired. Alcoholics (in particular homeless bums) sometimes buy the metho anyway and cop the poisonous effects, more specifically, the blindness.