Avian influenza — known informally as avian flu or bird flu — refers to "influenza caused by viruses adapted to birds." The type with the greatest risk is highly pathogenic avian influenza (HPAI).

"Bird flu" is a phrase similar to "swine flu," "dog flu," "horse flu," or "human flu" in that it refers to an illness caused by any of many different strains of influenza viruses that have adapted to a specific host. All known viruses that cause influenza in birds belong to the species influenza A virus. All subtypes (but not all strains of all subtypes) of influenza A virus are adapted to birds, which is why for many purposes avian flu virus is the influenza A virus.