The application provides definitions and synonyms from various dictionaries, Wikipedia articles and a glossary of Apple-related terms.

The application will perform an incremental search to show any matching headwords or forms and will try to bypass spelling errors.

However, the user can add new words to the macOS system-wide spell checker, which uses its own lexicon.

Mac OS X 10.5 (Leopard) added the Japanese-language dictionary Daijisen, Progressive English to Japanese and Progressive Japanese to English dictionaries, and the 25,000-entry thesaurus "Tsukaikata no Wakaru Ruigo Reikai Jiten" (使い方の分かる類語例解辞典), all of which are provided by the Japanese publisher Shogakukan.

In addition, dictionaries were also added for French (Multidictionnaire de la Langue Française for Macs sold in Europe, or Les Éditions Québec Amérique for Macs sold in America), German (Duden), Spanish (Vox), and Chinese (Standard Dictionary of Contemporary Chinese).