What is Semantic Layer?
A semantic layer is a shared set of business definitions, such as what counts as revenue or an active customer, applied consistently to every report.
Semantic Layer explained
It sits between raw tables and the people asking questions. Instead of each analyst rewriting 'net revenue' slightly differently, the layer defines it once, along with the tables and joins needed to compute it.
That consistency is what lets different tools and different people arrive at the same number.
Example
The layer defines 'active customer' as one with a paid order in the last 90 days, and every dashboard that shows active customers uses that rule.