Definition

Idiom is a set expression made up of two or more words; it functions as a unit of meaning which cannot be predicted from its literal meaning of its component words. 作为一个意义单位使用的，意义不能从其独立的组成部分得出的一种表达法。

For example, the idiom “show the white feather” means “to show fear”, and the meaning of this idiom has nothing to do with “feather”.

9. 1. Characteristics of idioms

Semantic unity语义的整体性

Each idiom is a semantic unity. The semantic unity can be reflected by the fact that the meaning of an idiom is very often not the total sum of the meanings of the constituent words. 

The semantic unity can also be shown in the illogical relations between the literal meanings of the constituent words and the meaning of the idiom.

Structural stability结构的固定性

Structural stability means that the structure of an idiom usually remains unchangeable. In other words, the constituent components of an idiom can not, generally speaking, be replaced.

The constituents of idioms cannot be changed or replaced. 习语的成分不能替换。

The word order cannot be inverted or changed. 习语的词序不可更换。

The constituents of an idiom cannot be deleted or added to, not even an article.习语的构成成分不可增减，即使是冠词也不行。

Ｍany idioms are grammatically unanalysable. 很多习语在语法上是不可分析的。