Is a customer discount for professional services an expense or income account?

In a single member LLC, if I want to provide a discount on professional services to certain customers when I do their invoice, I know I should set up an account for the discount, in the chart of accounts. 

What I'm not sure of is which type of account it should be. Is it an expense account? An income account? Can't find anything anywhere that tells me how the discount is accounted for.

The discount might be due to the volume of work they do with my company, or because they asked for 'a little help' on a tough job for them. Or due to loyalty or the length of our working relationship.

Thanks!