Synonyms are a feature in Algolia that allows users to specify which words or expressions should be considered equal, helping search engines provide more relevant results. To set up synonyms, users can choose from four types: identical terms, subset terms, matching records with placeholders for model numbers, and considering one-way synonyms as typos. Unlike most search engines, Algolia's typeahead search feature allows synonyms to match prefixes, providing a smoother user experience. By making the matched word clear in the result set, users can understand why the engine returned relevant results and avoid confusion. This powerful feature is available for users to learn more about in the Algolia documentation.