A way to measure a user's unique identity. This measure uses deduction or inference based on a rule or algorithm which is valid for that server. For example, the combination of IP address and user agent can be used to identify a user in some cases. If a server receives a new request from the same client within 30 minutes, it is inferred that a new request comes from the same user and the time since the last page request was spent viewing the last page. Also referred to as an inference.