Source : Webster's Revised Unabridged Dictionary (1913)

Superior \Su*pe"ri*or\, a. [L., compar. of superus being above,
   fr. super above, over: cf. F. sup['e]rieur. See {Super-}, and
   cf. {Supreme}.]
   1. More elevated in place or position; higher; upper; as, the
      superior limb of the sun; the superior part of an image.

   2. Higher in rank or office; more exalted in dignity; as, a
      superior officer; a superior degree of nobility.

   3. Higher or greater in excellence; surpassing others in the
      greatness, or value of any quality; greater in quality or
      degree; as, a man of superior merit; or of superior
      bravery.

   4. Beyond the power or influence of; too great or firm to be
      subdued or affected by; -- with to.

            There is not in earth a spectacle more worthy than a
            great man superior to his sufferings. --Spectator.

   5. More comprehensive; as a term in classification; as, a
      genus is superior to a species.

   6. (Bot.)
      (a) Above the ovary; -- said of parts of the flower which,
          although normally below the ovary, adhere to it, and
          so appear to originate from its upper part; also of an
          ovary when the other floral organs are plainly below
          it in position, and free from it.
      (b) Belonging to the part of an axillary flower which is
          toward the main stem; posterior.
      (c) Pointing toward the apex of the fruit; ascending; --
          said of the radicle.

   {Superior conjunction}, {Superior planets}, etc. See
      {Conjunction}, {Planet}, etc.

   {Superior figure}, {Superior letter} (Print.), a figure or
      letter printed above the line, as a reference to a note or
      an index of a power, etc; as, in x^{2} + y^{n}, 2 is a
      superior figure, n a superior letter. Cf. {Inferior
      figure}, under {Inferior}.

Source : WordNet®

superior conjunction
     n : (astronomy) the alignment of the Earth and a planet on the
         opposite side of the sun