It is vague because an edge is simultaneously the absence of something and the presence of many things at once. The absences are that:
anything you read in a book is not an edge
anything you read here is not an edge
anything you find in a charting application is not an edge
anything a vendor wants to sell you is not an edge
anything ANYONE shares with you is not an edge.
The multiple presences are even more confusing because all of the presences work some of the time, but none of the presences work all of the time. I give away nothing to tell you what traders who HAVE an edge will confirm. Intraday, a presence is:
a particular time slot
a particularly shaped twitch in price, usually invisible to the uninformed
a particular pattern of trades on the tape
a particular pattern of price in multiple time frames
proximity of price to secret support or resistance levels
particular patterns in the first derivatives of price or volume.
You can test everything you can think of, and nothing works by itself. If you try to test any two things together, you will think you found something, but it was a fluke of the finite sample sequence.
But when you SEE in sequence first one thing, then a second thing, and finally a third thing, AND you have the courage to act irrationally, THEN you have an edge. And that edge is untestable, because it occurs so rarely. As rarely as any other combination of three things occurs out of a universe of thirty of more posible events. So you have to know the ensemble of all the significant events, and have charts that help you see their rare confluences. Personally I think it's too much fucking work, but I have nothing better to do in my declining years than waste my time imagining I see edges and taking the trades.