A payment gateway is a merchant service provided by an e-commerce application service provider that authorizes credit card or direct payments processing for e-businesses, online retailers, bricks and clicks, or traditional brick and mortar. Wikipedia

What is “Online processing”:

Online processing is an automated way to enter and process data or reports continuously as use as the source documents are available. A good example of online processing is bar code scanning. When you buy a shirt at Target, the bar code gets scanned at the register. This shirt (source document) is immediately updated in Target’s inventory system as being sold. It is also updated in cost and sales reports. The online processing system continuously updates the entire accounting system. myaccountingcourse