Flip-flops are open summer shoes, usually rubber, with a thin sole and a V-shaped strap that goes between the big toe and the next toe. That strap is the whole “upper”: there is no back, no laces, nothing round the heel. You slide in and walk. The name is the sound they make on a hard floor — flip, flop, flip, flop. People wear them at the beach, by the pool, in hot weather, or to the shop when they cannot be bothered with real shoes. They feel casual, even a bit lazy; you would not wear them to a formal dinner or a job interview. One shoe is a flip-flop; people almost always talk about a pair of flip-flops. In American and British English this is the ordinary word. In Australia the same shoes are often called thongs — which, confusingly, means a kind of underwear in American English. Though actually, newspapers also use flip-flop as a picture for a politician who keeps changing sides: first this way, then that way, like the shoe slapping back and forth. That is a different use of the same sound-word. Here we mean the shoes.