Denatured alcohol is ethanol with added adulterants that make it useless for consumption but still useful for industrial processes. This is done in order to make it exempt from taxes that apply to potable alcohol.
There are diverse industrial uses for ethanol, and therefore literally hundreds of recipes for denaturing ethanol. Typical additives are methanol, isopropanol, methyl ethyl ketone, methyl isobutyl ketone , denatonium, and even aviation gasoline.
In this sense of the word, denatured means "a specific property of ethanol, its usefulness as a beverage, is removed". The ethanol molecule is not denatured in the sense that its chemical structure is altered.
See also methylated spirit.