Don

Don can stand for different concepts:
It is one of the main rivers in Russia, see Don River (Russia)
It is a name, see Donald (given name) for people with this name
Don (honorific), an honorific used in Spanish, Portuguese, Italian and Filipino
2,5-Dimethoxy-4-nitroamphetamine is abbreviated DON; it is a hallucinogen
Vomitoxin, also known as DON, is a toxin produced by different fungi
6-Diazo-5-oxo-L-norleucin, is an amino acid, that is tested as a chemotherapy against cancer. It is known as DON.
Nitrogen is sometimes referred to as DON