Definition of Flank opening

A flank opening is a chess opening played by White and typified by play on one or both flanks (the portion of the chess board outside the central d and e files). White plays in hypermodern style, attacking the center from the flanks with pieces rather than occupying it with pawns. These openings are played often, and 1.Nf3 and 1.c4 trail only 1.e4 and 1.d4 in popularity as opening moves.