Slippage
The difference between the price a trade was meant to fill at and the price it actually filled at.
Slippage happens when the market moves between the moment an order is sent and the moment it is filled, or when a broker’s pricing differs slightly from another broker’s at the same instant.
It matters most for very short trades and for entries during fast-moving or thin markets, because the fill’s cost is a bigger share of a short trade’s typical profit than of a long-held trade’s.