A trading edge in the financial markets can be described as a set of conditions that when present, give a higher probability of a trade working than not working.
An example of an edge could simply be identifying when the market is trending, in which case you base your trades on the direction of the trend. If you take trades in the direction of the trend, you have a higher probability of producing profitable trades.
Despite having an edge, there will be losing trades as well as winning trades. Trading in the direction of a trend does not guarantee a winning trade, merely a higher probability of a trade working out. The first trade that you take in the direction of the trend could be a losing one. However, taking a series of trades in the direction of the trend is likely to result in trades that win.
Source:
https://en.tradimo.com/learn/trading-strategies/trading-with-an-edge/
See also:
https://www.investopedia.com/articles/active-trading/022415/vital-importance-defining-your-trading-edge.asp
Also referred to as just an 'Edge'
I would like to add to this that an edge can be several things and not only a higher % of a positive outcome.
Having the fast computer in the world, or the fastest connection, could be an edge, for the time you have
that and the general public does not have that. It gives you and 'edge' in a sense that you are faster then
the rest of the crowd.
While not an exhaustive list, an edge is something that puts you ahead of the crowd.
This can be a statistically proven trading setup (for simplicity lets call this an algo)
This can be a mathematical model
This can be a synthetic data-feed
This can be a technology component
This can be personal skills, insight or industry knowledge
This can be 'insider information' (although that is illegal)
Bottom line, this is an 'advantage', you have and that allows you to make consistently money, as you are ahead of the mass.