What is an Idiom?
A group of words established by a usage as having meaning not deducible from those of the individual words.
Idioms help us to express our feelings and thoughts in a more colorful and illustrated way.
Idioms are a type of figurative language, which means they are not always meant to be taken literally. Idioms express a particular sentiment, but they do not literally mean what the individual words themselves mean.
An idiom is a saying that is specific to a language. For example, an idiom in English does not translate to an idiom in Spanish.
A group of words established by usage as having a meaning not deducible from those of the individual words (e.g. over the moon, see the light).
Facts:
1000´s
There are thousands of idioms, occurring frequently in all languages. In the English language alone, it is estimated that there are at least twenty-five thousand idiomatic expressions.
Literally
Many idiomatic expressions, in their original use, were not figurative but had literal meaning.
For instance, spill the beans (meaning to reveal a secret) has been said to originate from an ancient method of democratic voting, wherein a voter would put a bean into one of several cups to indicate which candidate he wanted to cast his vote for.
If the jars were spilled before the counting of votes was complete, anyone would be able to see which jar had more beans, and therefore which candidate was the winner. Over time, the practice was discontinued and the idiom became figurative
Key
Idioms are usually presumed to be figures of speech contradicting the principle of compositionality. That compositionality is the key notion for the analysis of idioms is emphasized in most accounts of idioms.
Idioms Web Quest 