Q. What is a DOI number?
DOI stands for Digital Object Identifier. According to APA Style, a DOI number is "a unique alphanumeric string . . . to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when your article is published and made available electronically."
In APA citations for journal articles, you will usually include a DOI number instead of a URL.
All DOI numbers begin with a 10 and contain a prefix and a suffix separated by a slash. It might look like this: doi:0000000/000000000000 or http://dx.doi.org/10.0000/0000.
For more on DOI numbers from the APA style manual, see this blog post from APA:
Some of the library databases provide DOI numbers for articles. If the DOI is unknown, you can also use the site CrossRef to look up numbers: