In the highest upvoted answer to Where does the $i$ come from in the Schrödinger equation? the author writes the following equation:
$$ U^\dagger U=(\mathbb I+\epsilon^* A^\dagger)(\mathbb I+\epsilon A)=I+\epsilon^*A^\dagger+\epsilon A+\mathcal O(\epsilon^2) $$
What does that last term mean? It looks a lot like the big O notation from asymptotic analysis.