Spread
The gap between the price a broker will buy at and the price it will sell at; the base cost of every trade.
Every position pays the spread once on entry, since the trade opens at the worse of the two prices. A strategy’s true edge has to clear this cost on every single trade before it clears anything else.
Spreads are not constant through the day: many brokers widen them sharply around the daily rollover, so a strategy that trades heavily in that hour pays more than its average-spread statistics suggest.