Normally an ADO .NET connection string uses "server" or "data source" to specify the machine name \ sql server instance, your application will be connecting to.
data source = .\sql instance name
server = .\sql instance name
SqlConnection conn = new SqlConnection(@"data source = .\sql2012;
integrated security = true;
database = AdventureWorks");
But there are few more ways to specify the same
address = .\ sql instance name
addr = .\ sql instance name
network address = .\ sql instance name