(astronomy) a more or less irregular star cluster containing tens to thousands of stars

(Britain, Australia) a can opener.

A device used to open tin cans, usually by slicing the lid off. / (roller derby) A shoulder hit to the chest, usually accomplished while moving from a crouched to a standing position.

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

An uncovered fire.

(literally) Something that causes the eyes to open, or that opens the eyes. / (informal) Something that provides a sudden insight, or makes something clear that was previously mysterious.

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