I think it's just a matter of semantics. An edge can be very slight edge...like beating the S&P 500's annual returns by 0.5% yearly with a little less volatility.
As for the original question, I'm not sure if you can define it. The only possible way is to actually trade until you have a sample with statistical significance that has included many types of market conditions (bull, bear, low vol, high vol, flat, etc.).
"Edge" is kind of a "sure thing".... like when an insider gets to buy stock before its IPO @ $.01/share. That's edge! Or when a desired person is recruited to a company and given stock options @ $.50/share while the stock is currently at $20. Retail traders get no such "edges".