From Traderpedia

Definition: A traders' edge is whatever technique or method he employs in his trading that puts him ahead of the field and makes him profitable over a statistically significant sample of trades.

The "Edge" itself will derived from the cumulative success of whatever method the trader employs to pick his entries and exits, but a the size of the Edge is definable mathematically through such calculations (after all costs are deducted) as profit factor, risk reward factor and average trade value.