This definition seems a bit dry and doesn't really tell anything about the function of idioms in English language. Without idioms English would lose much of its variety and humor both in speech an writing. The background and etymological origins of most idioms is at best obscure. This is the reason why a study of differences between the idioms of American and British English is somewhat difficult. But it also makes the cases, where background, etymology and history are known, even more interesting.
Biblical references are also the source of many idioms.
Sports terms, technical terms, legal terms, military slang and even nautical expressions have found their way to the everyday use of English language. In the old days English idioms rarely originated from any other form of English than British English. Nowadays American English is in this position. This is not the case with British English idioms which are not as widespread. It has to be remembered that it is hard to say which idioms are actively used in English and which are dying out or have already died.