Software Supply Chain
Dependency
Also: library, package dependency
A piece of code written by someone else that your project needs in order to run or build.
In more detail
Modern projects are mostly dependencies. A small web service can list a dozen packages directly and end up running hundreds, because each of those packages has dependencies of its own. Every one is code you execute with your program's permissions, so every one is part of your attack surface even though you never read it.