It's sometimes also described as a mock tender, scotch tender or petite tender.
Important point: the chuck tender IS NOT TENDER.
It's a heavily exercised muscle, which is used for walking around and supporting the weight of a 1,300-pound animal.
This is not to say that the beef industry or your butcher are deliberately misleading you. The chuck tender muscle is long and narrow, with a pointy tip at one end, which is the same shape as a beef tenderloin (the most tender cut of beef).
When Does "Tender" Not Mean Tender?
I want to think that butchers started calling it a chuck tender because it was shaped like a tenderloin, not because they wanted to mislead consumers.
Nevertheless, it's unquestionably misleading to have the word "tender" in the name of a tough, utterly untender piece of meat.
In a traditional 7-bone chuck roast, you'll get a cross section of the supraspinatus muscle, but since you're braising it, you won't have a problem.
But these days, butchers are pulling apart the beef chuck into all kinds of subprimes and merchandising the resulting steaks and roasts as things like flat iron steaks, Denver steaks, chuck eye steaks and so on.
Thus we have the chuck tender, which otherwise wouldn't exist.
Chuck Tender: Very Expensive Pot Roast
I recently saw petite tender steaks at the grocery store priced at $9.99/lb. Nowhere did it indicate that these steaks were from the chuck. If you didn't know, you might think they were tenderloin steaks.
Again, it's misleading. Which is not to say you shouldn't buy the chuck tender. It's just important to know what it is, and how to cook it.
Braised, chuck tender steaks will be tender and succulent and full of flavor. Roasted or grilled, however, they'll be tough and chewy.
Having said that, $9.99/lb is a lot to pay for what is essentially going to be pot roast. If you want to make a classic, beefy pot roast, you can usually find a chuck blade roast or 7-bone roast for $3.99/lb.