ACoS

Advertising cost of sales: your ad spend divided by the sales those ads were credited with, shown as a percentage.

You run Sponsored Products or Sponsored Brands, Amazon attributes any purchase made within the click window back to the ad, and the console divides what you spent by that attributed revenue. It is the default lens every Amazon campaign gets judged by, and it moves the moment your bids, conversion rate, or price move.

A campaign returns ad-attributed sales of five times its spend. Divide spend by sales and the ACoS is 20 percent.

Lower is not automatically better. ACoS only counts the sales the ad itself got credit for, so it ignores the organic rank those same clicks build. Teams chase a smaller number by cutting bids, volume drops, organic slips, and total profit falls even as the ACoS on the report looks healthier.

It measures efficiency on attributed sales, nothing more. Read it next to your break-even, not against zero.