Aji amarillo is a cultivar of capsicum Baccatum that only refers to one specific pepper in Peru/Bolivia. This pepper (aji amarillo) is used in a miltitude of dishes, plus sold at every supermarket, market, and made in a paste form throughout Peru/Bolivia. There is only one pepper that is the Aji Amarillo, and it is just as ubiquitous in Peru as the Jalapeño is in Mexico.
"Aji amarillo" literally means "yellow pepper" in South American Spanish, so any yellow pepper could be called an aji amarillo depending on the context.
The best part, is that the actual aji amarillo is orange, and gets it's name from the color it changes dishes when cooked with, which is of course, yellow.
Here are a few commercial bags of the EXACT same Capsicum Pubescens cultivar being sold as aji amarillo, because it's so damn tasty, everyone wants it in their kitchen.