Idiom

An idiom, also known as an idiomatic expression, is a phrase whose meaning cannot be deduced from a literal understanding of the words involved and which cannot always be translated word for word into another language. As they involve whole phrases, not even the context is enough. Instead, the whole phrase has a fossilised figurative meaning, a "figure of speech", that is understood only by those who have previously learned its meaning. Consequently they can be quite difficult, not to say impossible, for non-native speakers.

For instance, many sporting terms, from baseball or cricket, can be completely opaque for students; even native speakers may be confused unless they have a good working knowledge of the sport in question.

British newspapers are fond of using such expressions to make puns in their headlines - a habit sure to confuse the unwary.

i. Idioms can be very formal or very informal:
 * It’s raining cats and dogs!
 * To whom it may concern;
 * She was over the moon at the news!;
 * We’ll have to throw in the towel;
 * He just wanted to be a sleeping partner;
 * One for the pot;
 * Let’s have a quick one / have one for the road;
 * His name rings a bell!

They can be mythological/biblical, literary, military, nautical, or sporting, etc., in origin and many come directly or indirectly from Greek or Latin:
 * We have to find his Achilles heel;
 * It’s in the lap of the gods;
 * The die is cast = jacta alea est;

ii. Sayings, maxims or proverbs are expressions which generally offer advice or wisdom:
 * Actions speak louder than words;

iii. clichés and omissible phrases are expressions which some people may consider “irritating” because they are used too often:
 * Better late than never!;
 * Easier said than done!;
 * All things considered,...;
 * As a matter of fact,...;

For teachers who are interested in creating themed lists of idioms in class, a list of idioms associated with a particular subject can be created by using The free dictionary.