The act or process of adding, especially the process of computing with sets of numbers so as to find their sum.
Something added, such as a room or section appended to a building.
[Middle English, from Old French, from Latin additiō, additiōn-, from additus, past participle of addere, to add; see add.] ad·di'tion·al adj., ad·di'tion·al·ly adv.