What does "$R_{++}$" mean in "$x \log(x)$ is convex function either on $R_{++}$ or on $R_{+}$"?

Question

$x \log(x)$ is convex function either on $R_{++}$ or on $R_{+}$.

$R_{+}$ is a common symbol that I know, but I don't know what $R_{++}$ is referring to.