What is the sociological definition of international organization?

It is a group of persons representing or natural from more than one country who having a common purpose have grouped under a legally organized structure for achieving that common goal.

In common usage, the term 'International Organization' refers usually to Intergovernmental Organizations and therefore the above definition will be slightly changed to:

It is a group of persons acting as representatives of diverse governments who having a common purpose have grouped under a legally organized structure for achieving that common goal.