3rd party

Third party just means any software or add-ons that aren't made by the developers of the software into which you're installing it. Or for sites, it means apps/scripts/themes/other stuff that aren't made by the site owners. ex. For Facebook, most apps aren't made by the Facebook devs. They're made by other companies or users, so they're third-party apps. ex. Or for Firefox, their extensions aren't made by Mozilla, so they're also considered third-party.

3rd party refers to any software which you install in your OS other than which is pre-installed or comes as a feature of your Operating System. But this assumption is not valid for a the patches,updates or fixes provided by the manufacturer of your operating system. So, your operating system manufacturer becomes 1st party and if you download/purchase any software from other company/vendor, then that software is said to be 3rd party software.