Definition
Headline rate vs effective rate
A headline rate is the number in the advert, such as 2.6% plus 10 cents. Your effective rate is what you actually paid: total fees divided by total card sales. The two drift apart when monthly fees, gateway fees, chargebacks or higher interchange cards are added in.
The best rates for credit card processing are the ones that give the lowest effective rate on your own sales. Comparing headline rates alone can point you at the wrong provider.