Execution is the completion of a buy or sell order in the market. An order is executed when it is matched, filled, and recorded at an agreed price.
Execution quality depends on price, speed, liquidity, order type, broker routing, market conditions, and slippage. Fast execution counts most in volatile markets, where the price can move between the moment you send an order and the moment it fills.
Execution is the act of completing the order; the fill is the result, the actual price and quantity you receive. Slippage is the gap between the price you expected and the execution price, and it widens when the market moves fast or liquidity thins.
You place a market order to buy 1 lot of EUR/USD.
The order fills at 1.0852.
That is the trade execution, because your buy order has been completed at a confirmed price and size.