An active ingredient in unfinished form intended for manufacture into finished dosage form drug products. "Bulk" does not mean "quantity." Rather it means "raw" chemical ingredient. Because a bulk drug is unfinished, it does not carry FDA-approval.
Although a bulk drug might be produced by a facility "registered" by the FDA, registration just puts the drug into an FDA "catalog" of all U.S. drugs that are commercially distributed. Registration does not imply that FDA has reviewed or approved the bulk drug manufactured in the establishment. Registration means the facility that can be inspected by FDA, but this is not the same as a drug being approved by the FDA.
A drug whose manufacturer has demonstrated safety, efficacy, and product quality to the U.S. Food and Drug Administration for the labeled indication. Both "pioneer brand name" drugs and generic drugs are FDA-approved.
FDA maintains a list of approved animal drugs (Green Book) and approved human drugs (Orange Book).
Any drug that is manipulated based on a licensed practitioner's prescription, but not in accord with an FDA-approved label, to meet the medical needs of a specific patient. Compounded drugs are extemporaneously prepared drugs that lack FDA-approval. Compounded drugs undergo no approval for safety or efficacy, and rarely undergo any scientific evaluation.