An "orphan drug" is a pharmaceutical that has been abandoned or neglected during its development because it is seen as having only a limited potential for profit. Some of these drugs have a limited target population or treat a rare disease, thus limiting its moneymaking potential (but of course not its potential benefit to society).
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.