Definition:Big-O Notation/Also known as

Also denoted as
In analytic number theory, sometimes Vinogradov's notations $f \ll g$ or $g \gg f$ are used to mean $f = \mathcal O \left({g}\right)$.

This is clearer for estimates leading to typographically complex error terms.

Some sources use an ordinary $O$:
 * $f = O \left({g}\right)$