Irregular chess opening

In chess, irregular opening is a traditional term for any opening considered unusual or unorthodox. In the early 19th century it was used for any opening not beginning with 1.e4 e5 (the Open Game) or 1.d4 d5 (the Closed Game). As opening theory has developed and openings formerly considered "irregular" have become standard, the term has been used less frequently.
Because these openings are not popular with chess players, the standard opening references such as Modern Chess Openings (MCO) and Encyclopedia of Chess Openings (ECO) do not cover them in detail.
Usage of the term.
While the term has frequently been used in chess literature, its meaning has never been precise and has varied between writers. 
One of the earliest references to "irregular openings" in chess literature was made by William Lewis in his 1832 work "Second Series of Lessons on the Game of Chess". Lewis classified openings under the headings "King's Bishop's Game" (1.e4 e5 2.Bc4), "King's Knight's Game" (1.e4 e5 2.Nf3), "Queen's Bishop's Pawn Game" (1.e4 e5 2.c3), "King's Gambit" (1.e4 e5 2.f4), "Queen's Gambit" (1.d4 d5 2.c4) and "Irregular Openings" (all other openings). Lewis comments that the irregular openings are "seldom played, because they are generally dull and uninteresting". Among the openings he analyzes under this heading are the French Defence and English Opening (both now considered standard), Bird's Opening and a few 1.d4 d5 lines without the Queen's Gambit. Lewis assigns no names to these openings.
Carl Jaenisch, who was an early advocate of the French and Sicilian defences, rejected this use of the term "irregular", saying that openings should rather be classified as "correct", "incorrect" or "hazardous". In "The Chess-Player's Handbook" (1847), for many years the standard English-language reference book on the game of chess, Howard Staunton accepted Lewis's overall classification system while tacitly acknowledging Jaenisch's objections. He wrote "Those methods of commencing the game, in which the first or second player moves other than (1.e4 e5 or 1.d4 d5) are usually designated "Irregular". Without assenting to the propriety of this distinction, I have thought it advisable, for the sake of perspicuity, to adopt a general and well known classification in preference to arranging these peculiar "débuts" under separate and less familiar heads." Under this heading, Staunton considers the French Defence, Sicilian Defence, Scandinavian Defence, Owen's Defence, Dutch Defence, Benoni Defence, Bird's Opening and English Opening.
Examples.
Unusual first moves by White.
The vast majority of high-level chess games begin with either 1.e4, 1.d4, 1.Nf3, or 1.c4. Also seen occasionally are 1.g3, 1.b3, and 1.f4. Other opening moves by White, along with a few non-transposing lines beginning 1.g3, are classified under the code "A00" by the Encyclopaedia of Chess Openings and described as "uncommon" or "irregular". Although they are classified under a single code, these openings are unrelated to each other.
The openings classified as A00 are:
The Nimzowitsch-Larsen Attack (1.b3, ECO code A01) and Bird's Opening (1.f4, ECO codes A02–A03) have also been described as "irregular", particularly in older books.
Unusual responses by Black.
Openings in which Black makes an unconventional response to 1.e4 are classified as B00 (King's Pawn Game). Included in this code are:
Of these, 1...Nc6, 1...b6, 1...a6, and 1...g5 have received the most theoretical attention. Tony Miles famously used 1...a6 to defeat Anatoly Karpov.