So, this is pure curiosity. I've heard the term 3rd party, as in, "3rd party library" for quite a while.
I wonder why we say 3rd and not 2nd (or 4th)?
Who are the first and second parties and where did the term come from?
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You, the developer, are the first party.
The customer/consumer of your application is the second party.
Anyone else that supplies libraries, icons, or other resources is a third party (in the sense of "third" meaning some other outside party).
I think it's from contract language.
You, the 1st party, may create a contract with someone, the 2nd party, to undertake some piece of work. They may be an expert in one area of the project, but not in another and so they may choose to sub-contract that to a 3rd party.
Edit to add - In contract law you need two parties to create a contract, you will generally think of yourself as the first party and the other party to the contract as the second, but that's just a matter of perspective. One is not labelled as such. Anyone not directly involved in the contract is then a third party.
First party is the user of the software, second party is the producer or developer, third party is another producer/developer who adds to the second party's work already.
The current answer chosen is incorrect as @jimwise already indicated.