From NixNet

The word "instance" is used throughout this wiki. This is an extension of the way the term is used in Computer Science when talking about a process that has been created by a program. The process is an instance of that program. Within this wiki, it refers to a piece of open source software that is running on a particular server. Because the software is open source, it might be running on a hundred different servers owned and operating by a hundred different individuals with different rules and policies. These are called instances.