# What is G and what is H? What is the difference in the way they are defined?

Both ${\displaystyle G(s)\,}$ and ${\displaystyle H(s)\,}$ are notations for transfer functions. Sometimes we use ${\displaystyle G(i\omega )\,}$ (or ${\displaystyle H(i\omega )\,}$) to refer to the "frequency response" if the system is asymptotically stable; it can be obtained by evaluating ${\displaystyle G(s)\,}$ at ${\displaystyle s=i\omega \,}$.