The Vector Sum of Two or More Vectors Is Called What?

Answer

It is called the resultant vector, or simply the resultant.

If several vectors are $\mathbf A_1,\mathbf A_2,\ldots,\mathbf A_n$, their resultant is

$$\mathbf R=\mathbf A_1+\mathbf A_2+\cdots+\mathbf A_n.$$

The resultant is a vector, so it has both magnitude and direction. It represents the single vector with the same combined effect as the vectors being added. In component form,

$$R_x=\sum_i A_{i,x},\qquad R_y=\sum_i A_{i,y},\qquad R_z=\sum_i A_{i,z}.$$

If the component sums all equal zero, the resultant is the zero vector.