A website gets ten thousand hits a day. The owner is thrilled. Then she checks the analytics and finds that nine thousand of those hits came from one person refreshing the page. The number that matters is unique visitors: the count of distinct individuals who arrive at least once during a given period. Hits count requests. Unique visitors count people, or at least devices.

Measurement is tricky. A cookie can identify a browser, but one person may use three devices. A household may share one laptop. Privacy laws in Europe require consent for tracking, so some users become invisible. Analytics platforms use IP addresses, device fingerprints, and login data to estimate uniqueness. No method is perfect. Treat the number as a close approximation, not gospel.

Common periods

• Daily unique visitors: useful for news sites and daily deals.
• Monthly unique visitors: the standard for advertising rates.
• Annual unique visitors: rare, but useful for libraries and archives.

Advertisers pay based on reach. A site with one million monthly uniques can charge more than a site with one million pageviews from a thousand people. The distinction matters because attention is not the same as audience.