OneLake is the single storage layer of Microsoft Fabric. Every Fabric workspace stores its lakehouse and warehouse data there, mostly as Delta tables, and other engines can read it through a storage API compatible with Azure Data Lake Storage. Shortcuts let OneLake reference data held elsewhere without copying it.
For example, a Fabric workspace for sales might hold a lakehouse named sales whose tables are folders of Delta files in OneLake. A Spark notebook writes to those tables, the lakehouse's SQL analytics endpoint lets analysts query them with T-SQL, and a Power BI semantic model can read them in Direct Lake mode without importing a copy. A shortcut in the same lakehouse can point at a folder in an Azure Data Lake Storage account or another workspace, so that data appears as a table without being copied into OneLake.
Every Fabric tenant has one OneLake, organized by workspace and then by item, such as a lakehouse or a warehouse, and its permissions follow Fabric's workspace roles. Because tables are stored in the open Delta format, engines outside Fabric that understand Delta can read them through OneLake's storage endpoints, given the right access. The single copy is the point: the different parts of Fabric work on the same files rather than moving data between them.
Things to watch: Delta features that one Fabric engine writes may not be supported by another, capacity limits apply across the workloads that share a capacity, and shortcuts change where data physically lives, which matters for governance and cost. See also lakehouse, medallion architecture, and Unity Catalog for the governance layer on Databricks.
For a data engineer, the main difference from storage on other platforms is that OneLake is provisioned and governed by Fabric itself: there is no storage account to create, and the same workspace permissions apply to notebooks, warehouses and reports. See also Delta Lake.
OneLake holds the lakehouse tables that Emolysis's Fabric connector samples, through Spark, for local tests. Microsoft Fabric is planned; its connector is tested only on simulated Fabric services. See Connecting Microsoft Fabric and the Microsoft Fabric platform page.