Addition : Addition is the mathematical operation that describes increasing a number by an amount equal to a second number. The mathematical symbol for addition is the plus sign $+.$ The term addition is also used for a generalization of this basic operation on numbers to functions, vectors and matrices.

Imaginary part : The imaginary part of a complex number $a+bi$ is defined as $bi.$ The real part is defined as $a.$

Imaginary unit : The imaginary unit is the number $i=\sqrt{-1}$ with the property $i^2=-1.$ It is used in the construction of complex numbers of the form $a+bi.$