HTAP is used to describe the capability of a single database that can perform both online transaction processing (OLTP) and online analytical processing (OLAP) for the purpose of real-time operational intelligence processing. The term was created by Gartner in 2014.
In the SQL Server world you can think of it as: In-memory analytics (columnstore) + in-memory OLTP = real-time operational analytics. Microsoft supports this in SQL Server 2016 (see SQL Server 2016 real-time operational analytics).
I’m not completely sold on HTAP yet, particularly once you get to high-scale OLTP systems doing hundreds of thousands of transactions per second. That said, there’s always more and more pressure to get data available for analytics faster and faster.