What geometrical shape is a cylinder split in half?

Answer 1

a rectangle.

if u spilt a cylinder down the middle vertically you get a rectangle

If you split it in half parallel to the ends, it cuts the cylinder into two cylinders (the cut surface being a circle).
Cut at any angle between parallel to the ends and going through the point where the side meets the end produces an ellipse.
Cut at any angle between the point where the sides meet the ends and perpendicular to the ends produces a four sided shape with each side an arc, opposite sides being equal.