Avian flu is any flu caused by a virus adapted to birds.

It is also called bird flu, avian influenza and bird influenza.

The only known such viruses are Influenza A viruses. All subtypes (but not all strains of all subtypes) of this species are adapted to birds, so for many purposes avian flu virus is Influenza A virus (the "A" does not stand for "avian").

However most people use "avian flu" to mean H5N1 which is the subtype of the species Influenza A virus that is the world's major flu pandemic threat.

So "avian flu", "bird flu", "avian influenza", and "bird influenza" all mean the same thing; but are all used to mean these very different things:

flu:

• flu from Influenza A viruses which are adapted to birds (the technically correct meaning)
• the flu acquired from any Influenza A viruses
• the H5N1 flu (flu acquired from H5N1)

virus:

• Influenza A viruses which are adapted to birds
• the Influenza A viruses
• the H5N1 subtype of the Influenza A virus.

Detailed explanation of terms

"Flu" is short for "influenza". The two words mean exactly the same thing. No difference in meaning, only a difference in spelling and pronunciation.

"Bird" and "avian" used as an adjective in front of either mean exactly the same thing. No difference in meaning, only a difference in spelling and pronunciation.

So that gives us four ways of saying the same thing.

All four mean (1)a disease called a flu (2)from an infection from a virus adapted to birds (which includes no viruses other than Influenza A viruses, which includes all subtypes of Influenza A viruses, but does not include all strains of Influenza A viruses).

These four terms do not mean the virus itself, although people often use it that way anyway. These four terms do not mean just any Influenza A virus flu. Some Influenza A viruses have adapted to animals other than birds.