Fill is a trading term that refers to the completion of an order to trade a financial asset. When an order has been completed, it is often referred to as ‘filled’.
Usually, for an order to be filled, certain parameters will have to be met. Those parameters vary depending on the type of order being given. Market orders, for example, are filled at the best available price as quickly as possible, whereas GTC (good-till-cancelled) orders will remain unfilled until the price set by the trader is met.
When a broker is filling your order, it is often referred to as the order being executed.