What does $R^{\times}$ mean for a ring?

Question

What does it mean for $a \in R^{\times}$, where $R$ is a ring?

I cannot seem to find the definition of $R^{\times}$ anywhere (partially because I do not know what it is called).

Answer 1

$R^\times$ is used to denote the group of multiplicative units of $R$, i.e. $$ R^\times = \{\,x\in R\mid \exists y\in R\colon xy=yx=1\,\}.$$