A DOI, or Digital Object Identifier, is a unique alphanumeric string that identifies content and provides a persistent link to its location on the internet. It is assigned by the publisher when an article is published and made available electronically. The purpose of a DOI is to provide a permanent web address (URL) for an article or document, which will help readers easily locate it from a citation. A DOI is like a Social Security number for the article youre citing - it will always refer to that article, and only that one.

Here are some key points about DOIs:

• All DOI numbers begin with a 10 and contain a prefix and a suffix separated by a slash.
• The prefix is a unique number of four or more digits assigned to organizations, while the suffix is assigned by the publisher and was designed to be flexible with publisher identification standards.
• Not all articles or journals use DOIs; if you cant find a DOI, you can use the persistent URL in your citation instead.
• A DOI is a permanent ID that, when appended to http://dx.doi.org/ in the address bar of an internet browser, will lead to the source.
• If youre off-campus when you do this, youll need to use a URL prefix in front of the DOI to gain access to full-text journal subscriptions.
• APA style recommends the use of DOIs in citations for e-journal content.
• If no DOI is available and you retrieved the journal article online, you should include the persistent URL.

In summary, a DOI is a unique identifier assigned to an article or document that provides a permanent web address for it. It helps readers easily locate the document from a citation and is used in many citation styles for e-journal content.