A synonym is a word, morpheme, or phrase that has the same or nearly the same meaning as another word, morpheme, or phrase in a given language. Synonyms are different words that have the same or similar meanings and can be used in every part of speech, including nouns, verbs, adjectives, adverbs, and prepositions. The standard test for synonymy is substitution: one form can be replaced by another in a sentence without changing its meaning. Synonyms can be used to improve word choice or to avoid overusing the same word. They can also express a nuance of meaning or be used in different registers of speech or writing. Different technical fields may appropriate synonyms for specific technical meanings. Synonyms are often included in dictionaries along with the definition of the word as a means to guide language learners in their quest for meaning. Some synonym dictionaries implicitly distinguish between synonyms and near-synonyms, offering separate lists which differ according to denotation. Thesauruses, which focus on synonyms and often antonyms, have existed since the Thesaurus of English Words and Phrases was first published by British lexicographer Peter Mark Roget in 1852. Synonyms can be used as a tool to foster “elegant variation” to tackle monotony during written production.