同义词或者更学术性的称呼同义异形是世界上各种语言都存在的一种现象。它指的是表达的意义相同或相近，但是表达形式不同的词汇。例如厕所和“茅房”就是不同词汇表示同一种东西。

不仅词汇有同义现象，不同语法结构的句子也可以表示同一个意义。例如：

同义词之间的差别主要有感情色彩、理性意义、语法特点、各地习惯的不同。理性意义的差别又分别体现为范围、性状、程度等方面的不同。

A synonym is a word or phrase that means exactly or nearly the same as another word or phrase in the same language. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek syn (σύν) ("with") and onoma (ὄνομα) ("name"). An example of synonyms are the words begin, start, and commence. Words can be synonymous when meant in certain senses, even if they are not synonymous in all of their senses. For example, if we talk about a long time or an extended time, long and extended are synonymous within that context. Synonyms with exact interchangeability share a seme or denotational sememe, whereas those with inexactly similar meanings share a broader denotational or connotational sememe and thus overlap within a semantic field. Some academics call the former type cognitive synonyms to distinguish them from the latter type, which they call near-synonyms.