A repository that stores large volumes of data in its original format, whether structured, semi-structured, or unstructured. The schema is applied when reading the data (schema-on-read).
In DP-900 the typical foundation of a data lake in Azure is Azure Data Lake Storage Gen2. OneLake, Microsoft Fabric's data lake, is built on ADLS Gen2.
Data lake is a concept that can show up on the exam. Check the full guide to see it in context.