The Collaborative International Dictionary of English v.0.48:

Libration \Li*bra"tion\ (l[-i]*br[=a]"sh[u^]n), n. [L. libratio:
   cf. F. libration.]
   1. The act or state of librating. --Jer. Taylor.
      [1913 Webster]

   2. (Astron.) A real or apparent libratory motion, like that
      of a balance before coming to rest.
      [1913 Webster]

   Libration of the moon, any one of those small periodical
      changes in the position of the moon's surface relatively
      to the earth, in consequence of which narrow portions at
      opposite limbs become visible or invisible alternately. It
      receives different names according to the manner in which
      it takes place; as: (a) Libration in longitude, that
      which, depending on the place of the moon in its elliptic
      orbit, causes small portions near the eastern and western
      borders alternately to appear and disappear each month.
      (b) Libration in latitude, that which depends on the
      varying position of the moon's axis in respect to the
      spectator, causing the alternate appearance and
      disappearance of either pole. (c) Diurnal or parallactic
      libration, that which brings into view on the upper limb,
      at rising and setting, some parts not in the average
      visible hemisphere.
      [1913 Webster]