Why is it called a 3rd party?

Why is it called a 3rd party?

It’s a term that’s often used in Windows-centric development: the first and second parties are me (or you), and Microsoft; and the third party is anyone else: Sometimes it means the customer or end user (e.g. “”if we get a ‘redistributable’ from Microsoft, that means that we can redistribute it to ‘third parties'””)