session/visit

Official IAB definition:

"A single continuous set of activity attributable to a cookied browser (or if registration-based an Internet user) resulting in one or more pulled text and/or graphics downloads from a site without 30 consecutive minutes of inactivity. Establishing a session/visit can involve making assumptions about user activity and is subject to measurement limitations such as dynamic addressing and cookie deletion. The click measurement organization should consider these difficulties when creating rules for determination of sessions/visits."