REVERT

REVERT - Switches the execution context back to the caller of the last EXECUTE AS statement.

SELECT SUSER_NAME(), USER_NAME(); -- sa, dbo

EXECUTE AS USER = 'myUser';

SELECT SUSER_NAME(), USER_NAME(); -- myUser, myUser

REVERT;

SELECT SUSER_NAME(), USER_NAME(); -- sa, dbo

http://msdn.microsoft.com/en-us/library/ms178632.aspx