You should specify if you want a physical/mathematical meaning of Quadratic NLO (QNLO) activity or molecular system. So, in general a QNLO signal is obtained directly from the Second Harmonic Generation (SHG) and the obtained coefficients are expresed in pm V-1. I hope this will be helpful to you.

i want to know quadratic NLO property in physics? Basically i am working in the field of crystal growth, in most of the papers have title of quadratic nonlinear optical material or single crystal. But i couldnt able to understand that the term "quadratic" illustrates material which exhibit large second harmonic generation or some other meaning. If material having large SHG shall we term it as quadratic material.