In $\mathbb{R}^3$, the equation $x = 6$ (with no restriction on $y$ and $z$) defines a plane — specifically the plane parallel to the $yz$-plane at distance 6 from the origin. In $\mathbb{R}^1$ it's a point; in $\mathbb{R}^2$ it's a line; in $\mathbb{R}^3$ it's a plane.