Wiktionary
n. A mattress that is inflated with air, and used as a temporary or permanent bed.
WordNet
n. a mattress that can be stored flat and inflated for use
Wikipedia
An air mattress is an inflatable mattress/ sleeping pad.
Due to its buoyancy, it is also often used as a water toy/flotation device, and in some countries, including the UK, is called a lilo ("Li-lo" being a specific trademark).
Usage examples of "air mattress".
I got in and lay down and Morley, I could hear the maniac blowing up his ridiculous air mattress so he could lay beside me, but the moment he'd done so, he started at once to turn over and heave and sigh, and around the other side, and back toward me, and around the other side, all under the ice-cold stars and loveliness, while Japhy snored, Japhy who wasn't subjected to all the mad wiggling.
It was flat, about the size of an air mattress, with a hooded panel at one end.
She was lying face down on an air mattress with the brassiere of her bikini unfastened.
She was lying face down on an air mattress with the brassiere of her Bikini unfastened.
It was just blowin' up that goddamn air mattress that laid him out.
Sloane grinned at us all as we hauled in our third load of moss and began to blow up an air mattress.
There was an air mattress in there, and a cache of food and water.
Kelp's air mattress had not leaked, he was over there sleeping like a baby.
Dreamy and warm, he rolled over to pull her close for some serious snuggling, only to find himself alone on the air mattress.