The [[Complex Conjugate|Conjugate]] of a [[Quaternions|Quaternion]] is an [[Operation|operation]] similar to the [[Complex Conjugate]], where all non-[[Real Numbers|real]] [[Axis|axis]] has its [[Scalar]] flipped.
$\huge \begin{align}
q &= a+bi+cj+dk \\
q^{*} &=a-bi-cj-dk
\end{align}$
The conjugate $^{*}$ is [[Distributive Property|distributive]] over addition.
Multiplying any quaternion $q$ by its conjugate $q^*$ equals the [[Distance]] of $q$ to the origin $0$. This is also used to define the square of the [[Vector Magnitude|magnitude]] of $q$.
$\huge \begin{align}
qq^{*} = q^{*}q = |q|^{2} =a^{2}+b^{2}+c^{2}+d^{2}
\end{align}$