Source : Webster's Revised Unabridged Dictionary (1913)

Wane \Wane\, v. i. [imp. & p. p. {Waned}; p. pr. & vb. n.
   {Waning}.] [OE. wanien, AS. wanian, wonian, from wan, won,
   deficient, wanting; akin to D. wan-, G. wahnsinn, insanity,
   OHG. wan, wana-, lacking, wan?n to lessen, Icel. vanr
   lacking, Goth. vans; cf. Gr. ? bereaved, Skr. ?na wanting,
   inferior. ????. Cf. {Want} lack, and {Wanton}.]
   1. To be diminished; to decrease; -- contrasted with {wax},
      and especially applied to the illuminated part of the
      moon.

            Like the moon, aye wax ye and wane. Waning moons
            their settled periods keep.           --Addison.

   2. To decline; to fail; to sink.

            You saw but sorrow in its waning form. --Dryden.

            Land and trade ever will wax and wane together.
                                                  --Sir J.
                                                  Child.

Waning \Wan"ing\, n.
   The act or process of waning, or decreasing.

         This earthly moon, the Church, hath fulls and wanings,
         and sometimes her eclipses.              --Bp. Hall.

Source : WordNet®

waning
     adj : (of the moon) pertaining to the period during which the
           visible surface of the moon decreases; "after full moon
           comes the waning moon" [ant: {waxing}]
     n : a gradual decrease in magnitude or extent; "the waning of
         his enthusiasm was obvious"; "the waxing and waning of
         the moon" [ant: {waxing}]