Also, as well as. For example, They study their instruments and, in addition, theory and music history.
in addition to. Over and above, besides, as in In addition to a new muffler, the truck needs new brakes. [c. 1900]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.