资料来源 : Webster's Revised Unabridged Dictionary (1913)

   4. (Math.) A quantity greater than any assignable quantity of
      the same kind.

   Note: Mathematically considered, infinity is always a limit
         of a variable quantity, resulting from a particular
         supposition made upon the varying element which enters
         it. --Davies & Peck (Math. Dict.).

   5. (Geom.) That part of a line, or of a plane, or of space,
      which is infinitely distant. In modern geometry, parallel
      lines or planes are sometimes treated as lines or planes
      meeting at infinity.

   {Circle at infinity}, an imaginary circle at infinity,
      through which, in geometry of three dimensions, every
      sphere is imagined to pass.

   {Circular points at infinity}. See under {Circular}.