Ask
Ask is the lowest current price someone is willing to sell at in the order book.
Definition
Ask is the lowest price currently offered by sellers in the order book. The difference to bid is the bid ask spread.
Why it matters
If you enter a position immediately, you often pay ask. That makes ask a conservative reference for comparing your fair price to the market.
Common pitfalls
Using mid only: Mid can understate the cost of crossing the spread.
Assuming ask size: Top of book may not represent available size.