Wiktionary
semi-open game
n. (alternative case form of Semi-Open Game English)
Wikipedia
Semi-Open Game
A Semi-Open Game is a chess opening in which White plays 1.e4 and Black breaks symmetry immediately by replying with a move other than 1...e5. The Semi-Open Games are also called Single King Pawn, Asymmetrical King Pawn, or Half-Open Games (or Openings), and are the complement of the Open Games or Double King Pawn Games which begin 1.e4 e5.