The DOI (Digital Object Identifier) is a unique and persistent identifier for a journal article, conference paper, book chater, thesis, or other digital content.

The DOI uniquely identifies a specific article, paper, or chapter. DOIs are used on articles published by commercial and open access publishers, professional societies, institutions, and other organizations.

The DOI system was developed by the International DOI Foundation. DOIs are typically assigned as part of a deposit process into the CrossRef database. The CrossRef system stores searchable metadata -- including the DOI -- for each article/chapter.

DOIs are often printed within articles (typically on the first page) or in references to articles. Actual DOIs are in the form of an alpha-numeric string such as 10.1246/bcsj.73.1653. The DOI is used to redirect to the appropriate publisher site where the full-text of the article is available for display. The DOI is "resolved" by using the dx.doi.org prefix site. So, as an example:

http://dx.doi.org/10.1246/bcsj.73.1653

takes the user to the Bulletin of the Chemical Society of Japan web site where the full-text link to the specific article can be found.