A synonym is a word that has the same meaning, or a very similar meaning, as a different word in the same language. Two words that are interchangeable in a sentence because each word means exactly the same thing, or very nearly the same thing as the other, are synonyms.
An example of a pair of synonyms are the words "easy" and "simple." Both words are understood to have the same meaning and can be used interchangeably to describe the same thing. The word "synonym" is a noun, and "synonymous" is the adjective derivative. For example, the word "easy" is synonymous with the word "simple."