Transferring Data from OLTP to OLAP databases Systems that have both OLTP and OLAP databases periodically move data from the OLTP database to the OLAP database, so that reports can be generated based on the OLAP database.

What's the official term for this process and is there an overview of this process online?
It is mostly referred as ETL process (extraction, transformation, and load). 

Here are a link of MSDN article on Transforming OLTP Data to OLAP Data Warehouses. It is an old article but the same concept applied.