Idioms are words, phrases, or expressions that are either grammatically unusual, as in, “Long time, no see!”, or their meaning cannot be taken literally, as in, “It's raining cats and dogs!”
This expression does not mean that cats and dogs are falling from the sky, but it is a metaphorical expression (word picture) that means that it is raining very heavily.
Idiom (Latin: idioma, “special property”, f. Greek: δίωμα – idiōma, “special feature, special phrasing”, f. Greek: διος – idios, “one’s own”) is an expression, word, or phrase that has a figurative meaning that is comprehended in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made. There are estimated to be at least 25,000 idiomatic expressions in the English language.
In linguistics, idioms are usually presumed to be figures of speech contradicting the principle of compositionality; yet the matter remains debated In phraseology, they are defined in a similar way as a sub-type of phraseme whose meaning is not the regular sum of the meanings of its components. John Saeed defines an “idiom” as words collocated that became affixed to each other until metamorphosing into a fossilised term. This collocation—words commonly used in a group—redefines each component word in the word-group and becomes an idiomatic expression. The words develop a specialized meaning as an entity, as an idiom. Moreover, an idiom is an expression, word, or phrase whose sense means something different from what the words literally imply. The idiom “beating around the bush” means to hint or discuss obliquely; nobody is literally beating any person or thing, and the bush is a metaphor. When a speaker uses an idiom, the listener might mistake its actual meaning, if he or she has not heard this figure of speech before. Idioms usually do not translate well; in some cases, when an idiom is translated into another language, either its meaning is changed or it is meaningless.