Pronunciation: (u-dish'un),[key] —n. 1. the act or process of adding or uniting.
2. the process of uniting two or more numbers into one sum, represented by the symbol +.
3. the result of adding.
4. something added.
5. a wing, room, etc., added to a building, or abutting land added to real estate already owned.
6. Chem.a reaction in which two or more substances combine to form another compound.
7. in addition to, as well as; besides: In addition to directing the play, she designed most of the scenery.