Session capping is a term in online advertising

Online advertising

Online advertising is a form of promotion that uses the Internet and World Wide Web to deliver marketing messages to attract customers. Examples of online advertising include contextual ads on search engine results pages, banner ads, blogs, Rich Media Ads, Social network advertising, interstitial...

 that means restricting (capping) the amount of specific advertising sessions a visitor to a website

Website

A website, also written as Web site, web site, or simply site, is a collection of related web pages containing images, videos or other digital assets. A website is hosted on at least one web server, accessible via a network such as the Internet or a private local area network through an Internet...

 is allowed to have.
It greatly diminishes the amount of money the advertiser can earn.

Here, a session is defined as the total amount of time one visitor spends on a single website, regardless of how many pages he or she visits within that time. When the visitor closes his browser, or enters an entire different website, the session ends. When the visitor returns to the website, a different advertising session can be started. See also session (computer science)

Session (computer science)

In computer science, in particular networking, a session is a semi-permanent interactive information interchange, also known as a dialogue, a conversation or a meeting, between two or more communicating devices, or between a computer and user . A session is set up or established at a certain point...

 and

Session capping is somewhat similar to frequency capping

Frequency capping

Frequency capping is a term in advertising that means restricting the number of times a specific visitor to a website is shown a particular advertisement...

.

Using session capping, a specific advertising scheme can be applied to a website visit. For example, advertisements can be repeated indefinitely within a single session, or a specific order can be implemented.

http://www.talkphp.com/general/1077-understanding-life-session.html
http://javapapers.com/servlet-interview-questions/explain-the-methods-used-for-session-tracking/