Wiktionary
bubble level
n. A tool employing a chamber with a colored liquid and an air bubble, used to determine if a surface is level horizontally, vertically, or at a 45-degree angle.
Usage examples of "bubble level".
He set the automatic pilot for 60 mph, then engaged it while watching the bubble level.
With his eyes on a bubble level, his helmet hooked by wire phone to his mate, he nursed a control stick which let him keep the ship upright.
Place them on the doors, set the device level, using a bubble level, high on your outside wall, where there is a good distance to fall.
At a building-supply place, picked up a builders' protractor, a bubble level, and some duct tape.