Tabular models are in-memory databases in SQL Server Analysis Services, an OLAP server bundled with SQL Server by Microsoft.
From Books Online:
Tabular models are in-memory databases in Analysis Services. Using state-of-the-art compression algorithms and multi-threaded query processor, the xVelocity in-memory analytics engine (VertiPaq) delivers fast access to tabular model objects and data by reporting client applications such as Microsoft Excel and Microsoft Power View.
Tabular models have been available since SQL Server 2012.