Groundloop's definition may be technically correct, but I've never really considered 'yield' to be "the average price per seat sold" because that, as he says, doesn't give you any meaningful information as to the profitability of the flight. You certainly need to know that number but what you're actually looking for is the balance to obtain the highest possible volume against the highest average fare. That should result in maximimum revenue for the flight and it's actually achieving that which is so difficult.
I wouldn't really differentiate between the terms 'sector revenue' and 'total sector yield' in everyday usage, but it's quite an interesting subject to discuss.