From Wikipedia: "An identity document (also called a piece of identification or ID, or colloquially as one's '"papers"') is any document which may be used to verify aspects of a person's personal identity. If issued in the form of a small, mostly standard-sized card, it is usually called an identity card (IC). In some countries the possession of a government-produced identity card is compulsory while in others it may be voluntary. In countries which do not have formal identity documents, informal ones may in some circumstances be required.
In the absence of a formal identity document, some countries accept driving licences as the most effective method of proof of identity. Most countries accept passports as a form of identification."
For the U.S. : a Passport, a Passport Card or a Driver's License are valid. See http://en.wikipedia.org/wiki/Identity_document#United_States