Glossary
An application that ships as one unit but has clearly separated modules with real boundaries inside.
The modular monolith answers the experience that distributed systems carry their own cost: network failures, distributed transactions, operational effort per service. Draw the boundaries cleanly but deploy together, and you get clarity without that bill.
The practical benefit comes later: if a module really has to become its own service, the seam already exists. Going from module to service is short work. Going from tangle to service is a project.