a Cartesian coordinate system consisting of two perpendicular axes for graphing complex numbers, the real part of a number being plotted along the horizontal axis and the imaginary part along the vertical axis.

mathsa diagram in which complex numbers are represented by the points in the plane the coordinates of which are respectively the real and imaginary parts of the number, so that the number x + i y is represented by the point (x, y), or by the corresponding vector < x, y >. If the polar coordinates of (x, y) are (r, θ), r is the modulus and θ the argument of x + i ySee also amplitude (def. 5)

Word Origin for Argand diagram

C19: named after Jean-Robert Argand (1768–1822), French mathematician