Radial set

In mathematics, given a linear space ${\displaystyle X}$, a set ${\displaystyle A\subseteq X}$ is radial at the point ${\displaystyle x_{0}\in A}$ if for every ${\displaystyle x\in X}$ there exists a ${\displaystyle t_{x}>0}$ such that for every ${\displaystyle t\in [0,t_{x}]}$, ${\displaystyle x_{0}+tx\in A}$. Geometrically, this means ${\displaystyle A}$ is radial at ${\displaystyle x_{0}}$ if for every ${\displaystyle x\in X}$ a line segment emanating from ${\displaystyle x_{0}}$ in the direction of ${\displaystyle x}$ lies in ${\displaystyle A}$, where the length of the line segment is required to be non-zero but can depend on ${\displaystyle x}$.

...
Wikipedia