The standard definition of nonlinear susceptibilities in SI units is via a Taylor expansion of the polarization's reaction to electric field: P = P 0 + ε ε 0 χ χ ( 1 ) E + ε ε 0 χ χ ( 2 ) E 2 + ε ε 0 χ χ ( 3 ) E 3 + ⋯ ⋯ . (Except in ferroelectric materials, the built-in polarization is zero, P 0 = 0 .) The first susceptibility term, χ χ ( 1 ) , corresponds to the linear susceptibility described above. While this first term is dimensionless, the subsequent nonlinear susceptibilities χ χ ( n ) have units of (m/V).