A table format that adds a transaction log on top of Parquet files. It enables ACID transactions, updates, deletes, schema enforcement, and queries over previous versions (time travel).
In DP-900 it is the default table format in Microsoft Fabric and Azure Databricks lakehouses. OneLake stores data from almost every Fabric workload in Delta format.
Delta Lake is a concept that can show up on the exam. Check the full guide to see it in context.