BAdI (Business Add-In)
What it is
An SAP-provided, object-oriented enhancement point you implement to inject custom logic into a standard process — without modifying SAP code. In ABAP Cloud / S/4HANA, released BAdIs (and the key-user Custom Logic app) are the sanctioned hook.
Why it matters for Clean Core
BAdIs are the clean way to change standard behaviour: you plug into an extension point SAP guarantees, so your logic survives upgrades. Released BAdIs are a core part of developer extensibility.
Common pitfall
Using an old, unreleased enhancement technique (implicit/explicit enhancements, user-exits) where a released BAdI exists. Only released enhancement options are Clean-Core-safe — check the object's released enhancement points first.