The term off-label refers to the use of a drug for a medical condition other than for which it was officially approved and marketed.
In the United States, all drugs intended to treat an illness or medical condition must be approved by the Food and Drug Administration before they can be legally marketed. Marketing information for the drug will list one or more indications, i.e., illnesses or medical conditions for which the drug has been shown to be both safe and effective. Pharmaceutical companies are not allowed to promote a drug for any other purpose without formal FDA approval.
However, once a drug has been approved for sale for one purpose, physicians are free to prescribe it for any other purpose that in their professional judgment is both safe and effective; they are not limited to its official, FDA-approved indications. This off-label prescribing is most commonly done with older, generic medications that have found new uses but have not had the formal (and often costly) applications and studies required by the FDA to formally approve the drug for these new indications. However, there is usually extensive medical literature to support the off-label use.
Some drugs are used much more off-label than for their original, FDA-approved indications. A good example is the use of tricyclic antidepressants for neuropathic pain. This is an older class of antidepressants that is now rarely used on clinical depression because of side effects, but which are usually more effective on pain than the newer SSRI class of antidepressants.