Trading term
What is Execution?
Execution is the moment your order stops being an instruction and becomes an actual trade at an actual price. Brokers owe you a duty to seek the best result reasonably available when they do it.
Your order doesn't go straight to an exchange. Your broker decides where to send it, and there are usually several places it could go: the listing exchange, a competing exchange, a wholesaler that fills orders internally, or a dark pool. That routing decision is execution, and it determines the price you actually get.
Regulators treat it as a duty rather than a courtesy. US brokers owe a best-execution obligation and must publish routing statistics; in the UK and EU, MiFID II requires firms to take all sufficient steps to get the best possible result for a client, judged on price, cost, speed, likelihood of completion and size — not price alone. That list matters. The fastest fill and the best price aren't always the same route.
What you're measuring is slippage: the gap between the price you saw and the price you got. On a liquid stock in a calm market it's a fraction of a cent and hardly worth thinking about. On a thin stock, a fast market, or a large order relative to what's on the book, it can dwarf whatever you paid in commission.
For example
You send a market order to buy 500 shares with the offer showing $50.02. Your broker routes it to a wholesaler that fills you at $50.015 — half a cent of price improvement, $2.50 across the order. Send the same order into a news spike and you might fill at $50.14 instead, and the $60 difference never appears on any statement as a fee.
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Explore Premium →Why it matters to you
Execution quality is the cost that doesn't show up on your bill. A zero-commission broker still charges you, through the spread and through where it routes. The practical defence costs nothing: use limit orders where you can, be careful around the open and around news, and read your broker's routing disclosures — they exist precisely so this cost can be compared.
⚠ Best execution doesn't mean best price
The obligation is to seek the best overall result on a defined set of factors, not to guarantee the top of the book on every fill. A firm can meet it and still fill you a cent worse than the best quote if the route was faster or more likely to complete. Judge it on averages over many orders, not on the one fill that annoyed you.