This article uses corpus evidence to examine uses of the word synonym in two ways. First, it examines whether uses of synonym match common dictionary definitions of the word. This turns up both senses of synonym that are missing from general dictionaries and broadenings from the basic ‘sameness of meaning’ sense represented in most dictionaries. After reviewing user studies that discuss synonym searches, the article turns to the study of a web-derived corpus of text related to searching for synonyms. The corpus gives insight into the types of expressions that people seek synonyms for, the reasons they search for them, and how well thesauruses meet those needs. These are considered with reference to seven electronic thesauruses. The data indicate types of expression for which thesaurus treatment could be improved, including multiword, closed-class, and ‘vulgar’ expressions. Suggestions are made for future directions in electronic thesaurus design and usability research.