Execution Price
Execution price is the actual price you get filled at. With multiple fills, the correct single number is the VWAP across fills.
Definition
Execution price is the actual price at which your order (or parts of it) is filled. If your trade fills in multiple pieces at different prices, your effective execution price is the VWAP.
Why it matters
Execution price is the key input for measuring real costs. Effective spread is computed from execution price relative to the midquote.
Common pitfalls
Using the quote instead of the fill: Your fill can be worse than the displayed bid or ask due to limited depth.
Ignoring partial fills: Use VWAP when your order fills across multiple prices.