Expect to come across this popular question in business intelligence interviews. A semantic layer is a layer of abstraction between the underlying data sources and the tools and applications that access the data. It provides a common language and a consistent set of definitions and relationships that can be used to access and analyze data from multiple sources. A semantic layer is important for BI because it helps to hide the complexity of the underlying data sources and enables users to access and analyze data in a consistent way, regardless of the source.