over left right arrow cap x cap y passes through the centers of the front and back sides of the prism. What is the best name for over left right arrow cap x cap y ?

The line through the centers of the prism’s front and back sides is a line, not a plane. If the prism can be turned around this line and still match its original shape, the line is an axis of symmetry, so the best name is “axis of symmetry.”

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