(1) Having been altered in natural quality.
(2) Of, or pertaining to, a molecule (such as a protein or a nucleic acid) wherein its chemical structure is altered through chemical or physical means so that some of its original properties are lost or diminished.
(3) Of, or pertaining to, a food or drink (often an alcohol) that is made unfit for human consumption (in case of alcohol, by the addition of methanol) while its use for other purposes is retained.
A protein that is said to be denatured means that its tertiary structure is caused to unfold possibly by heat, alkali, acid, or by other chemical or physical means, making the molecule lose its original state and become biologically inactive.
A denatured nucleic acid such as DNA means that the paired strands have become separated into individual strands.
A denatured alcohol (ethanol) does not mean its structure is changed. It only means that its feature as a beverage has been removed by adding toxic and bitter tasting chemicals. That is why it is still useful for other purposes, such as a solvent.
Related term: denaturation