\begin{definition}[Definition:Resonance Frequency]
Consider a second order ODE in the form:
:$\dfrac {\d^2 x} {\d t^2} + 2 b \dfrac {\d x} {\d t} + a^2 x = K \cos \omega x$
for $a, b \in \R_{>0}$.
Let $b < a$, so as to make $S$ underdamped.
Let the forcing frequency $\omega$ be such that $S$ is in resonance.
Then $\omega$ is known as the '''resonance frequency'''.
\end{definition}