An ELT Glossary : Addition

Addition is a discourse relationship in which two or more propositions ("ideas") are seen as being "the same" in some way. 

If we accept the proposal, we'll need to dedicate an entire room to the new equipment. In addition, we'll have to take on at least four new staff.  ( = two disadvantages of accepting the proposal)

As well as being a  Detective Chief Inspector in the CID, he's also a well-known musician who has recorded several albums.  (= two achievements of the person described)

Addition can be indicated by :

Prepositions : besides, as well as, in addition to
Conjunctions : and, not only/but also, 
Adverbials : Moreover, Furthermore, In addition, Also, Besides, too, as well