ASK HN:There are 3rd party application why not 4 or 5 or n party application.
there are 3rd party application why not 4 or 5 or n party application.
The 2nd party is the customer, either you or the organisation you work for.
A 3rd party is anyone else.
Choice of indefinite rather than definite article in the last case is deliberate.
The terms come from law: most contracts are between two entities ("the first party" and "the second party") and any other entities not party to the agreement are referred to as "third parties".