It’s publicly exposing someone’s real name, address with a combination of other personal data on the Internet. This without the consent of that person.
So let’s say there was an argument between Ninjach and S3ns31.
Ninjach seeks revenge by looking up any clues on S3ns31’s real personal details.
The research is performed sometimes with basics internet research or OSINT techniques.
When Ninjach discloses those details against the will of S3ns31 on the internet, that would be referred to as doxxing.
There are cases where Journalists are being doxxed, Hackers or just innocent people being doxxed by mistake.