Amazon PPC glossary
Bid
Your bid is the most you are willing to pay for one click. It is a ceiling, not a price. What you actually pay is usually lower, and it is set by the auction.
Example
You set a $1.00 bid. When a shopper clicks, you might pay $0.72 or $0.91, but never more than $1.00.
Where it matters most: Book 1, The Amazon PPC Clarity Guide, in the chapter “Bid Optimization.”
Go deeper
The glossary gives you the one-line answer. The Amazon PPC Clarity Guide explains why it matters and what to do about it.