Definition
A fill is the execution of an order — when your buy or sell order is matched with a counterparty and the transaction is completed, you've received a fill.
Example
“I placed a limit order at $14.50 for 500 shares. Got a partial fill of 300 shares at $14.50, and the remaining 200 never filled as the stock gapped away from my price.”
Detailed Explanation
Getting filled sounds simple — you place an order, you get filled — but in practice it's more nuanced. Market orders fill immediately but at whatever price is available, which can differ from what you saw on your screen, especially in fast-moving markets. Limit orders give you price control but may result in partial fills (some shares filled, others not) or no fill at all if price never reaches your limit. Understanding which order type to use in which situation is a foundational skill.
Fill quality matters, especially for strategies involving many trades per day. If you're consistently getting filled at worse prices than expected — a form of negative slippage — it compounds across trades. Brokers differ significantly in execution quality. The speed of their routing, the smart order routing algorithms they use, and whether they sell order flow to market makers (payment for order flow) all affect whether you get best execution. Reviewing your actual fill prices versus your intended prices weekly can reveal execution issues worth addressing.
In fast markets — news spikes, earnings reactions, opening bell volatility — fills become unpredictable. A market order that takes half a second to execute might fill several ticks away from where you clicked. For this reason, many day traders prefer limit orders in illiquid or volatile conditions, accepting the risk of non-fill over the risk of terrible execution. The decision depends on how urgently you need the position and how much slippage you can stomach.