Search data leakage is the process by which when a user does a search on a search engine and then clicks on a link, the search terms are sent to the site that was clicked on and sometimes to an other site through third party tags. Search terms are passed through the referrer URL (HTTP referrer header).
On the landing site, search terms are used for web analytics purposes, product recommendation and site retargeting.
When the search keyword data is passed and sold to a third party, it is used for behavioral targeting and search retargeting.
Some search engines encrypt their URLs to limit search leakage. Since 2011, Google encrypts search data when a user has signed in.
See also data leakage.