The visit to a website from a unique IP address within a certain short period of time, usually 30 minutes. If within that time frame a user leaves and returns to a site, this is still only classed as one User Session, if however they leave and return after an hour, this may be considered a separate session. Subsequently, web site traffic can be measured from the amount of user sessions it achieves. A user session commences when an application is accessed and ends when the user quits the application.

See also:
Unique Visitor
Unique Page Views
Cost per acquisition (CPA)
Cost per click (CPC)
Reciprocal links
Ad rotation
Ad impression
Behavioural Targeting
Contextual Marketing
RSS
E-Marketing
User Session
Deep Linking