Average Price, commonly referenced as Average Fill Price, is the average price paid or received for a particular coin when there are multiple fills per order.
Calculated as an average price would be the sum would be:
Assuming you have 3 separate fills at three different price points
($ x qty) + ($ x qty) + ($ x qty) / (qty + qty + qty)
So, if you:
sell 100 @ $5.20
sell 50 @ $5.25
sell 50 @ $5.10
The mathematical sum would be
= (100 X $5.20) + (50 * $5.25) + ($50 @ $5.10) / (100 + 50 + 50)
= (520 + 262.50 + 255) / 200
= $5.1875 is the Average Price paid