If an encyclopaedia entry has a named author the format for a chapter in a book should be used with the addition of the encyclopaedia volume number.
AUTHOR(S) (Year) Title of chapter. In: AUTHOR(S)/EDITOR(S) (ed./eds.) Title, Volume (if applicable). Edition. Place of publication: Publisher, Pages. (use p. or pp.)
If there is no author then the title (e.g. Oxford English dictionary) should be used both within the text and in the reference list.
Title. (Year) Volume (if applicable). Edition. Place of publication: Publisher.
Note: If you are referencing from an online source use the standard referencing format for reference works as detailed above.