(idiomatic) To remain vigilant for someone or something. / (idiomatic) To remain vigilant for a possibly dangerous situation.

A problem with an easy solution.

An interior of a building constructed with few walls or obstructions, giving a communal aspect.

plural of open and shut case

plural of open book decomposition

(mathematics, topology) A decomposition of a 3-manifold into a link and a fibration over the circle by surfaces bounded by that link.

A museum where the exhibitions are located outdoors.

A method of aiming an artillery piece with the target in sight (direct fire), as opposed to indirect fire where the target is not visible.

plural of Semi-Open Game

A type of chess opening, in which White plays 1.e4 (moves their king's pawn to the fourth rank with their first move) and Black breaks symmetry by replying with a move other than 1...e5.