Create Nintex Constant Variables

First go to Central Admin then select Nintex Workflow Management there.

In that click on Manage constants then you will see below page to create new constant variable you required.

Click on New icon to create a new constant. Configure it according to your requirement.

there are five data types variables you can create. (String, Number, Date, Credential, Secure string)

eg: You want to store Nintex database connection string

Then you can test your variable by creating small sample Nintex workflow as follow.

Configure SQL action using your newly created Constant.