Check out "horizontal cylindric segment" in Wolfram Alpha Online. That is the correct term for the solid you are looking for.
Wolfram Alpha - "The solid cut from a horizontal cylinder of length L and radius R by a single plane oriented parallel to the cylinder's axis of symmetry (i.e., a portion of a horizontal cylindrical tank which is partially filled with fluid) is called a horizontal cylindrical segment."
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Semi-circle. (2D) Hemi-sphere (3D)
2D = semi circle 3D = hemisphere
a circle in 3D would acually be called a SPHERE
A sphere
A semi-ellipsoid.