The imaginary unit i, where i² = -1 - Complex arithmetic: addition, multiplication, division - The complex conjugate z* and the modulus |z| - Euler's formula: e^(iθ) = cos(θ) + i sin(θ) - The polar form of complex numbers: z = re^(iθ) - The fact that |ψ|² = ψ*ψ is always real and non-negative — this