Dedicated Administrator Connection (DAC) allows administrators emergency access to SQL Server via SQL Server Management Studio (SSMS) or via command prompt (SQLCMD utility) when SQL Server is not responding to normal connections, so that they can execute the diagnostic functions (typical set DBCC commands, DMVs and system stored procedures) to troubleshoot server-wide problems.

Checkout my article (Using a Dedicated Administrator Connection (DAC) on SQL Server) in which you will learn, how to obtain emergency access to a SQL Server Database Engine using the built-in Dedicated Administrator Connection (DAC) functionality.

This article is published on SSWUG.org.