libration

Libration is the back-and-forth oscillation of a system that remains trapped within a potential well, never accumulating enough energy to escape over the barrier. The pendulum swinging left and right without going over the top is librating.

The term comes from the Latin libra, a balance — the same root as the zodiac sign. In celestial mechanics it describes the apparent rocking of the Moon that lets us see slightly more than half its surface over time. In the phase portrait of a pendulum, libration orbits are the closed loops inside the separatrix: the system rocks back and forth, periodically reversing direction.

Libration contrasts with rotation, where the system has enough energy to pass over the potential barrier and keep going in the same angular direction indefinitely. The boundary between the two regimes is the separatrix.