Fill

A fill is what happens when your order actually gets executed in the market — you asked to buy or sell, and a trade occurred at some price and size. Until that happens, an order is just a request sitting in the market's order book waiting for someone on the other side to take it. Once it happens, you have a fill, and it's a fact: a specific quantity traded at a specific price.

Fills can be full or partial. A full fill means your entire order was executed at once — you wanted 500 shares and you got all 500 in one trade. A partial fill means only some of it went through, say 200 of the 500, with the remaining 300 still working (or cancelled, depending on your order settings). You can also get multiple fills for a single order, each at slightly different prices, if the market only had small amounts available at each price level as your order worked through it.

The nuance that trips people up is that "getting filled" doesn't mean getting filled at the price you expected. If you placed a market order (an order that says "execute now, whatever the price"), your fill price depends entirely on what was available in the market at that instant — it can be noticeably worse than the last quoted price during fast moves, which is called slippage. If you placed a limit order (an order that only executes at your chosen price or better), you won't get a fill at all unless the market actually trades there, which is why "no fill" is a common and normal outcome, not an error.

Traders talk about fills constantly because the gap between what you intended and what you actually got — your fill — is where real trading costs and real risk show up. A clean, quick fill at your expected price is unremarkable; a slow, partial, or badly priced fill is often the thing that turns a good idea into a bad trade.