# Definition
An antiunitary operator $\mathcal{O}$ is characterized by:
$
\mathcal{O}^\dagger \mathcal{O} = I
$
$
\mathcal{O} \lambda = \lambda^* \mathcal{O}
$
for some complex number $\lambda$.
# In Physics
The most prominent example is the [[time-reversal operator]].