Addition is a mathematical operation that involves combining two or more numbers to produce a SUM. The crossword solutions for the term 'addition' include ALSO (4 letters), SUM (3 letters), RIDER (5 letters), TOTAL (5 letters), HIKE (4 letters), BONUS (5 letters), ADDING (6 letters), SUMMATION (9 letters), ACCESSORY (9 letters), ACCUMULATION (12 letters), CONTINUANCE (11 letters), CONTINUATION (12 letters), ADD (3 letters), ELL (3 letters), GAIN (4 letters), WING (4 letters), ANNEX (5 letters), INCREASE (8 letters), and INCREMENT (9 letters). All of these words refer to the act of ADDING two or more numbers together.