Chapter 12: Vectors and the Geometry of Space - Section 12.1 - Three-Dimensional Coordinate Systems - Exercises 12.1 - Page 695: 8

circle with equation $y^2+z^2=1$ lying in the zy plane

Since, we have the points that are the intersection between the equation of cylinder $y^2+z^2=1$ and the plane $x=0$. The set of points will have the equation of a circle $y^2+z^2=1$ in the zy plane , origin $O$ as center and having circle of radius $1$ .