Edge in trading is simply the ability of the trader to be RIGHT consistently. The markets are always right and the bigger question is are you the trader on the right side or on the wrong side?
Casinos have an edge hence they are in business, it is the odds that are in their advantage to win over time. So it is fair to conclude the odds, the rules of payout and restrictions of bets in place are part of a system of controls. eg. Roulette. These rules are there for their own benefit and not the punter.
In blackjack, it is similar but it is based on cards yet there is a possiblilty of beating the system because you can count cards, it is based on the probability and understanding the odds and play it to your advantage and not theirs. Casinos have rules and the punter usually plays along the rules, if you do that, you lose.
In trading, an example would be riding the trends. The markets moves in a particular way and for the trader to win, it needs to see this beforehand so to establish a system to take advantage of it. That system would be defined by rules to allow the trader to enter and exit to complete the trade or trades. The example is simply following your system to ride the trends, some will lose but most will win, and for the ones that wins, they can win more because you are allowing it to run, just like a trend. The edge itself is that your winners will earn you a lot more than the losers so the advantage you have over time is you will win.
The casino rules are a system and the punter who does not have one will lose.
The markets is a system and the trader without a plan / system will lose.
How can you beat an enemy without knowing their weakness?