A much abused or misunderstood term. In the IAM context it generally refers to the particulars of an authentication. More specifically, it can mean:
Identity [1] - the established relationship between an Entity and a particular Registration (eg a regsistered user's EOI details). An Entity can have multiple Identities, usually one per Registration. An Identity may have multiple Accounts, usually one per environment or platform. Also see Entity.
Identity [2] - an instance of an Entity. A user (username and password), an account.
Identity [3] [lesser usage] - the identifier (username, customer number) used as a means of identifying an Entity. If this usage is adopted, then "Digital Identity" is the term to be used to mean the relationship between an Entity and a particular Registration, or the instance of an Entity (ie Identity [1] or [2] above). But this is regarded as a clumsy invocation that does not add to understanding or communication.

An identifier is issued by the registration authority. A particular identifier known as a GUID (Globally Unique Identifier) is generated in the identity repository, but is not usually issued to the entity.

Source: http://identityaccessman.blogspot.com/2006/08/identity-dictionary.html