Definition:Polynomial Ring/Notation

Notation
It is common to denote a polynomial ring $\struct {S, \iota, X}$ over $R$ as $R \sqbrk X$, where $X$ is the indeterminate of $\struct {S, \iota, X}$.

The embedding $\iota$ is then implicit.

Also see

 * Polynomial Ring is Generated by Indeterminate over Ground Ring, which justifies the notation $R \sqbrk X$.