Unique visits: A visitor is considered unique when she visits your Web site the first time during a given period of time. Depending on the metric — the measurement used to track visitors on your Web site — that’s used, a visitor might be considered unique the first time he visits your site in a 24-hour period, the first time in a week, or the first time in an hour. For AdSense, the unique visits measurement is used to help determine the click-thru rate for ads.
Click-thru rate (CTR) is the number of people who click an ad and are taken to the Web page designated for the advertisement. This page is usually a larger, more colorful ad, the opening page of a Web site, or a page that displays more information about the product or service featured in the ad.
Average click-through-rate (CTR): The CTR is the actual number of visitors who click through an ad on your Web site. This is important because you’re paid when users click your AdSense ads.
Average cost-per-click (CPC): The CPC is the amount that advertisers pay each time someone clicks one of their ads. This number varies widely and is dependent upon the cost of the keyword to which an ad is related. For you, as an AdSense publisher, the CPC is the basis for how much you’re paid.