(idiomatic) To begin firing (weapons) at something or someone.

An uncovered fire.

A problem that has been posed in a field of study such as mathematics or theoretical computer science, but whose answer is not yet known.

Something that is in its beta stage, or being tested.

Pertaining to a product sold by merchants at a discount, because the item was returned but found not to be defective.

plural of open problem

Alternative spelling of openwork

alert and vigilant

(informal) simple and obvious; easily decided

Taking place outdoors; alfresco