Glossary
Also: ACL
A translation layer between new and old, so that the quirks of the legacy system do not seep into new code.
Building next to a legacy system leaves you a choice: adopt its data model and inherit its baggage, or translate. The anti-corruption layer is the second answer. It speaks the language of the old system on the outside and the language of the new one on the inside.
The price is another layer to maintain. The gain is that the new side keeps a clean model and the old side can be switched off without everything shifting.