"Privacy" as used on this site is shorthand for the idea that personal data should be looked after, specifically data that is on the internet. It is also referred to as Data Privacy or Data Protection.
Personal data is defined in the local laws that govern this area, but is generally taken to be information about living individuals.
The laws governing this area vary from country to country. In Europe there are Data Protection laws which are then enshrined in laws in each of the member countries.