libration

noun

1. 1

   The act of librating.

2. 2

   The apparent wobble or variation in the visible side of the Moon that permanently faces the Earth, allowing observers on Earth to see, over a period of time, slightly more than half of the lunar surface.

3. 3

   (by extension) A similar rotational or orbital characteristic of some other celestial body.

4. 4

   A vibrational degree of freedom whereby a group of atoms partly rotates back and forth, similar to the Moon's motion.