A DOI (Digital Object Identifier) is a unique number assigned to an item that provides a persistent link to that item. Some citation styles encourage the use of a DOI in reference list citations.
You can either look for this number (not the same as an accession number) in the information about the article like this example from Academic Search Complete database:
Or try this free DOI lookup tool:
http://www.crossref.org/guestquery/
If you cannot locate a DOI for your item, check a citation manual for your style (MLA, APA) to see what to do instead or ask a librarian for help.
