is most commonly used. often though it's convenient to use trigonometric form, which is consequence of interpreting complex number (a,b) as a point on plane (so called 'complex plane') with coordinates a and b.
and angle φ such as:
this is trigonometric form. real number |z| we call modulus of complex number z, and φ it's argument, φ = arg z.
if z ≠ 0 and we assume that φ belongs to [0, 2*Pi) then trigonometric form is unambiguously set. we can write then: φ = Arg z.
See also: Complex Numbers Body.