HTAP is a hot topic in recent years. And it is coined by Gartner.
This page links to the slides on HTAP. It talks some basic problems on HTAP, such as What it is, and Why HTAP, then on the types of HTAP systems. Just as similiar concepts in storage systems like Unified storage, there are many design options for HTAP. And in real systems, there have many tradeoffs.

Link to the pdf: An Introduction to HTAP.