Fill

The price at which an instruction was actually executed, and the time at which that happened. For an order asking for immediate execution the fill is whatever the venue can do at that instant, which is not necessarily the price displayed a fraction of a second earlier. That difference is slippage, it runs in both directions, and it describes a price that moved while a message was in flight.

How the price is arrived at depends on the order. An instruction to deal immediately is matched against the best available interest at the moment it lands, and where the quantity resting at the best price is smaller than the order, the remainder is matched at the next prices available. That produces either a sequence of partial fills or a single volume weighted average, depending on how the provider reports it, and providers differ on which of those a client sees. An instruction with a price limit on it is filled at that price or better and never worse, so it carries no adverse slippage and carries instead the possibility of not being filled at all.

Everything downstream is calculated from the fill, not from the quote that was on the screen when the instruction was sent, and the two are different numbers often enough that the distinction is the whole point of the word. A confirmation that arrives slowly does not mean the execution happened slowly either: the timestamp on the fill is the record, and the trade confirmation and the account statement are where that record is kept.