From the Latin, "wave of the sea”,
it refers to a particular stop of the organ.
It is a "soul” stop, part of the fluctuating family.
It consists in two equal pipes, one of which, however, is slightly lower than the other.
By making them play simultaneously, the phase displacement between the two frequencies produces a particular physical phenomenon
called beating, which generates the specific “swaying"
and flute-like effect of the sound.