In APA, DOI stands for Digital Object Identifier, which is a string of numbers, letters, and symbols used to uniquely identify an article or document and link to it on the web. It is like a Social Security number for the article you are citing, and it will always refer to that article and only that one. The DOI is typically located on the first page of the electronic journal article, near the copyright notice. In APA format (7th edition), include the DOI for all works that have one. It goes at the end of your reference, and you should write the DOI as a hyperlink starting with https://doi.org/ . If your article has no DOI whatsoever, which may happen with older articles, simply omit this from the citation.