Here is Databricks page on that:
https://www.databricks.com/glossary/data-lakehouseThat wasn't very enlightening (at least not to me), so here is Google's page: https://cloud.google.com/discover/what-is-a-data-lakehouse
That page wasn't great either.
As best as I understand it, they are trying to add more structure to the data lake, and optionally ACID compliance, so that datawarehouse tools can talk to the lake directly.