Using the processing power in a user's computer to do cryptocurrency mining while visiting a website. Operating in the background, the user is unaware cryptojacking is taking place. JavaScript code in the Web page enlists the visitor's computer to do the mining, and although it only lasts as long as the visitor is on the site, the mining time accumulates if there are thousands of visitors every day.
The website owner is generally not the culprit; rather, a third-party has compromised the site by installing the cryptojacking routine. However, no matter how the routine was inserted into the site, the only thing that is stolen from the user is CPU processing cycles. See
crypto mining malware and
crypto mining.