Hi Steve,

Impersonation is a generic term for using the security settings of a user that you are not authenticated as. This term is used *both* for

(a) database users impersonating other database users (using the EXECUTE AS command, or executing stored procedures with the EXECUTE AS clause), which requires the aptly named IMPERSONATE permission - see also the link free_mascot posted; and

(b) the SQL Server service connecting to another instance under the context of an authenticated Windows user.

So both the first and the third answer are correct.