Definition from Wiktionary, the free dictionary
- A medicinal drug which is effective in the treatment of some disease(s), but which is not manufactured or marketed because the demand is insufficient to cover the costs of supply.
- Tragically, her disease is so rare that the only effective medicine is an orphan drug.
- (US, law) A drug as defined under the Orphan Drug Act (1983). (Can we add an example for this sense?)