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