Inflatable describes something designed to be filled with air until it becomes firm and holds its shape. An inflatable pool toy starts as a flat piece of plastic, but after you blow air into it or pump it full, it becomes a float you can ride on. Inflatable life jackets, mattresses, and bounce houses all work the same way: air pressure inside makes them stiff enough to use.

The beauty of inflatables is that they pack down small when you're not using them. An inflatable kayak can fold into a backpack, perfect for hikers who want to paddle across a mountain lake. An inflatable camping mattress takes up almost no space in your gear until you need it for sleeping.

The word can also be used as a noun: “We brought several inflatables to the pool party.” You'll see inflatables everywhere from hospitals (where doctors use inflatable casts for broken bones) to outer space (where NASA tests inflatable habitats for future Mars missions). Anything that goes from flat to functional by filling with air earns this useful name.