This book is about idioms, idiomatic expressions and their usage. Idiom is a group of words whose meaning is different from the meanings of the individual words: Let the cat out of the bag is an idiom meaning to tell a secret by mistake. Idiom is the kind of language and grammar used used by particular people at a particular time or place. Idiom is the style of writing, music, art, etc. that is typical of a particular person, group, period or place: the classical/contemporary/ popular idiom. Idiomatic is containing expressions that are natural to a NATIVE SPEAKER of a language, i.e. speaking fluent and idiomatic English.