As usual Al, you had me looking things up for even more information. You're a great motivator!
There is a very loose definition used by some that includes the mountain lion and cheetah. The one that I like was the one you found that only includes the Panthera species. The mountain lion is not a panthera, but a member of the puma family. The mountain lion not being able to roar was interesting because of the many sounds it does make. The one thing about the big cat definition that says mountain lions are not included because of their size however is not based in reality. The mountain lion is about the same size as a leopard and much bigger than a snow leopard. If it were to be included in the list of big cats, it would be in the middle according to size.