In what context? Third-party is “someone else”, so normally either:
- code in your codebase that is not yours and not controlled by you (think libraries, frameworks, scripts, calling APIs, etc).
- updates/add-ons for software that are released by parties that are not the original developer (like fan mods for games).
First-party is the opposite, but it’s very rarely used in common written/spoken English outside of descriptions of insurance.