And if there is a doubt about the fat that they have to be "vintage" (not currently in operation) here you have some sources (better than opinions). The EAA definition defines warbird for Oshkosh.
Gerry
http://www.warbirdsresourcegroup.org/ (in a way they define the term on their website when they talk about what they do
Wiki: Warbird is a term used to describe vintage military aircraft. Although the term originally implied piston driven aircraft from the World War II era, it is now often extended to include all military aircraft, including jet powered aircraft, that are no longer in military service. Vintage jet aircraft in flyable condition, however, are much rarer due to technical complexity.
Sometimes, the term "warbird" also applies to newly built replicas and reproductions of vintage aircraft, such as Allison V-1710 powered Yak-9s from Yakovlev, Me 262s built by the Me 262 Project and FW 190s by Flug Werk, and can, for earlier eras when military aircraft design was of a less complex nature, even include any one of a large number of different World War I and later military aircraft designs, up through the late 1930s in origin, sometimes powered by vintage engines from the era of the aircraft design being flown.
Restored warbirds are a frequent attraction at airshows. Highly modified as well as "stock" warbirds can also frequently be seen at air races, since late-war fighter planes are among the fastest propeller-driven planes ever built. Some of the most popular warbirds for races are the P-51 Mustang, the Hawker Sea Fury, the F8F Bearcat, and the T-6 Texan.
Oxford Essential Dictionary of the US Dictionary
n. a vintage military aircraft.
wordIQ.com:
Warbird - Definition
Warbird is a common term used to describe fighter aircraft from the World War 2 era. The term implies propeller-driven planes with high-powered, noisy engines, risky flight maneuvers and colourful camouflage paint schemes.
Restored warbirds are a frequent attraction at airshows. Souped-up warbirds can also frequently be seen at air races since late-war fighter planes are among the fastest propeller-driven planes ever built. The most popular warbirds for races seem to be the P-51 Mustang, the Hawker Sea Fury, and the F8F Bearcat.
The term is sometimes extended to include vintage bombers and jet fighters; however, most warbird aficionados will only accept a prop-driven fighter as a true warbird.
How about this:
According to EAA Definitions
Warbird = ALL ex-military aircraft.This specifically means an aircraft
that was operated BY the military of any country. This EXCLUDES civilian
equivalents of military aircraft, or developmental aircraft that have
never been operated in a military capacity.
wordnic.com has a good one also:
warbird
–noun
1. an aircraft formerly operated by the military, but now owned privately.
2. any aircraft decorated in military livery for display at airshows etc.
You can also look in:
American Heritage Dictionary
Century Dictionary
GNU Webster's 1913
WordNet
Elsewhere on the web