"Well-known reference books, such as major dictionaries and encyclopedias, are normally cited in notes rather than in bibliographies. The facts of publication are often omitted, but the edition (if not the first) must be specified. References to an alphabetically arranged work cite the item (not the volume or page number) preceded by s.v. (sub verbo, “under the word”; pl. s.vv.)"
To cite an online reference work, follow the above rules and add the title's URL or DOI to the citation.
To cite a less-known reference title, or if you need a citation with publication information, consult the section in this guide on citing Book Chapters.