addition - Addition; addition - Process of uniting two or more numbers or symbols in one sum. Simple addition is the adding of numbers, irrespective of the tilings denoted by them, or the adding of sums of the same denomination. Compound addition in arithmetic is the adding of sums of different denominations, as of pounds, shillings, and pence. This addition of all kinds of multiple quantities or symbols is performed according to the principles of compound addition. Thus, the addition of two algebraic imaginary quantities is effected by adding the real parts of the expressions to obtain the new real part, and the imaginary parts to obtain the new imaginary part.