#axis  a straight line through a body or figure that satisfies certain conditions
  supertype:  #line.location  a spatial location defined by a real or imaginary unidimensional extent
  subtype:  #coordinate_axis  one of the fixed reference lines of a coordinate system
     subtype:  #x-axis  the horizontal axis in a plane coordinate system
     subtype:  #y-axis  the vertical axis in a plane coordinate system
     subtype:  #z-axis  the third axis in a 3-dimensional coordinate system
  subtype:  #major_axis  the longest axis of an ellipse or ellipsoid; passes through the two foci
  subtype:  #semimajor_axis  one-half the major axis of an ellipse; the distance from the center of an ellipse to one end
  subtype:  #minor_axis  the shorter or shortest axis of an ellipse or ellipsoid
  subtype:  #semiminor_axis  one-half the minor axis of an ellipse
  subtype:  #principal_axis__optic_axis  a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted

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