To modify the molecular structure of a protein or other biological macromolecule, causing it to lose its original properties. Heating the egg whites will denature the proteins, causing them to solidify.
To add a substance to alcohol to make it unfit for drinking while retaining its other properties. The company is required to denature the ethanol before it can be used in industrial applications.
To alter the natural qualities of something, rendering it unnatural or less effective. Over-processing can denature the food, stripping it of its essential nutrients.