According to the Merriam-Webster dictionary (merriam-webster.com), a synonym is a word that has the same meaning as another word in the same language; for example, "small" and "little" are synonyms. A synonym can also be a word or phrase that embodies the same idea, such as "tyranny" and "oppression."
In a more scientific context, a synonym may also designate two or more words in the same taxonomic group. A thesaurus is a good resource for finding synonyms. Synonyms are important for keeping writing lively and fresh and to avoid boring repetition. A word that holds the opposite meaning is an antonym; for example, hot and cold, up and down, etc.