What is the name of the data source when we use SQL Server Express Edition?

Answer:

It is .\SQLEXPRESS
It is specified in the connection string.
The complete connection is like this:

Data Source=.\SQLEXPRESS;AttachDbFileName=c:\abc.mdf;Integrated Security=True;
User Instance=True

It is presumed that abc.mdf file exists in c drive. abc.ldf must also be there.
Integrated Security is used to authenticate SQL Server Express Edition.
User Instance=true. It starts SQL Server Express Edition using the current user's
account.