Chicago (18th ed.) Citation Style Guide: Reference Works

"Well-known reference books, such as major dictionaries and encyclopedias, are normally cited in notes rather than in bibliographies....If a physical edition is cited, not only the edition number (if not the first) but also the date the volume or set was issued must be specified. 

References to an alphabetically arranged work cite the item (not the volume or page number) in quotation marks. Though the owrd "under" may be added for clarity, this manual no longer recommends using the Latin abbreviation s.v. (Chicago Manual of Style, 18th ed., 14.130).

An edition number is not required for continually updated resources. Provide a publication posted date or revision date (or an access date if those are not available" (Chicago Manual of Style, 18th ed., 14.131).

Bibliographic Form

Footnote Form

Shortened Form

"Online dictionaries and encyclopedias are normally cited in the notes rather than bibliographies...For continually updated resources, an edition number will usually be unnecessary; instead include a publication date or the date of the most recent update or revision" (Chicago Manual of Style, 18th ed., 14.131).

"Online dictionaries and encyclopedias are normally cited in the notes rather than bibliographies...For continually updated resources, an edition number will usually be unnecessary; instead include a publication date or the date of the most recent update or revision" (Chicago Manual of Style, 18th ed., 14.131-2).

In this example, the Dictionary of Canadian Biography has a print counterpart so the title is italicized. Wikipedia does not have a print counterpart so the title is not italicized (see Chicago Manual of Style, 18th ed., 14.103-4 for more information). 

"Time stamps may be included for frequently updated resources" (Chicago Manual of Style, 18th ed., 14.131).

Bibliographic Form

Footnote Form

Shortened Form

"For certain reference works - particularly those with substantial, authored entries - it may be appropriate to cite individual entries by author...Such citations may be included in a bibliography" (Chicago Manual of Style, 18th ed., 14.132).