An architecture that combines the flexible, low-cost storage of a data lake with data warehouse capabilities, such as tables, SQL queries, and ACID transactions, usually by using the Delta Lake format.
In DP-900: in Microsoft Fabric, a lakehouse stores files and Delta tables in OneLake, is processed with Spark, and exposes a read-only SQL analytics endpoint. Azure Databricks also provides lakehouses.
Lakehouse is a concept that can show up on the exam. Check the full guide to see it in context.