Compress means to squeeze or press something into a smaller space. When you compress a sponge in your hand, you force out the air and water, making it occupy less room. When you sit on your sleeping bag to stuff it into its storage sack, you're compressing it.

Air can be compressed into tanks for scuba diving. Scientists compress gases to study their properties. Computer files get compressed so they take up less storage space and transfer faster over the internet. The data gets squeezed down without losing the important information.

You might compress your thoughts when writing, fitting a long explanation into just a few clear sentences. A compression bandage presses against an injury to reduce swelling.

The opposite of compress is expand: when you release a compressed spring, it expands back to its original size. Many things in nature and technology rely on this back-and-forth cycle between compression and expansion, from your lungs as you breathe to car engines to the way sound waves travel through air.