Boundary
A line in the code where one part hands over to another: your code and the network, one module and the next, your code and a library. What crosses it should be checked and kept narrow.
A boundary tells an agent where its change should stop, and lets several agents work at once, one on each side. It is also where untrusted data comes in, so it is the one place that checking really has to happen.
Say it to your agent
“All calls to Stripe go through src/payments/stripe.ts. Nothing outside that folder may import the stripe package, and the types that cross the line are ours and not Stripe's. Add a structural test for it.”