Target audience
A specified audience or demographic group for which an advertising message is designed.

Tracking pixel (also Web beacon)
Single line of code which is used by a website or third-party ad server to track a user's activity, such as a registration or conversion (Source: IAB)

Traffic
The number of visits to a Web page or site.

Unique visitors
Number of unique users who visit a page or site (usually counted once per 24 hours).

View-through tracking
Ad tracking technology that measures actions occurring in the time after the initial impression.

Viral marketing
Advertising and/or marketing techniques that "spread" like a virus by getting passed on from consumer to consumer and market to market (Source: IAB)