Chicken tenderloin is another name for chicken tenders and chicken breast strips. Its not a legal standard and is used in a number of different ways in the food industry.
As described by the U.S. Department of Agriculture, a poultry tenderloin is the meat along the inside of the breast, close to the bone. The tenderloin is easily removed from the breast and contains a gristly tendon that must be removed before cooking.
According to the online food encyclopedia Practically Edible, the terms "chicken tender" and "chicken tenderloin" aren't legally enforceable definitions. They are the USDA's own description of general practices they have observed. In the USDA's own words, "The terms tender and tenderloin have been used for a number of years for muscles from the breast without a clear cut definition."
Chicken tender sometimes refers to ground-up chicken that is formed into strips before being breaded and fried. In some stores, butcher's may sell chicken tenderloins cheaply so they can avoid the difficult task of removing the tendon themselves. Other stores may use the term "tenderloin" as marketing and an excuse for higher prices.