Vacuum is a space that contains no air or other matter. A true, perfect vacuum is rare, though outer space comes close. In a vacuum, there's nothing to carry sound (which is why space is silent) and nothing to push against or breathe.

Scientists create partial vacuums in laboratories by pumping air out of sealed containers. These vacuums help researchers study how things behave without air resistance: a feather and a hammer dropped in a vacuum fall at exactly the same speed, something astronauts demonstrated on the Moon.

The word also refers to a vacuum cleaner, which got its name because it uses a pump to create a partial vacuum inside. This lower air pressure causes outside air to rush in through the nozzle, carrying dust and dirt along with it. When you vacuum the floor, you're using air pressure differences to pull particles into the machine.

People sometimes describe an empty or lonely feeling as a vacuum: “When my best friend moved away, it left a vacuum in my life.” This usage captures how a vacuum is fundamentally about absence, whether it's the absence of air molecules or the absence of something important.