(authoritative definition can be found at http://www.whitehouse.gov/sites/default/files/omb/memoranda/fy04/m04-04.pdf)
cite #desc Description of Assurance Levels
This [..] describes four identity authentication assurance levels for e-government transactions. Each assurance level describes the agency’s degree of certainty that the user has presented an identifier (a credential in this context) that refers to his or her identity. In this context, assurance is defined as (1) the degree of confidence in the vetting process used to establish the identity of the individual to whom the credential was issued, and (2) the degree of confidence that the individual who uses the credential is the individual to whom the credential was issued.