n.d. (no date)
Also called: n.d., undated
The abbreviation used in place of a year when a source carries no identifiable publication date.
When a source genuinely has no date, APA and Chicago author–date substitute “n.d.” wherever the year would go, in both the in-text citation and the reference entry: (Larsen, n.d.). MLA simply omits the date rather than marking its absence.
Look before you resort to it. Web pages often carry a date in the page source, a copyright line in the footer, or a “last updated” stamp; a PDF usually has one in its metadata. An access date is not a substitute for a publication date, but most styles want one for undated online sources precisely because the content may change.
A retrieval date and “n.d.” together are the honest signal that the source is unstable — which is itself information a reader can use.