1A group of more than two, or (sometimes more generally) of an indeterminate number of things, persons, etc.
2Specifically. Philosophy and Logic. A relative term containing more than two elements.
A set of energy levels of a vibrating molecule which would be degenerate if vibrations were harmonic but which actually differ minutely in energy owing to anharmonic interactions with different modes of vibration or rotation.
Mid 19th century. From poly- + -ad, after monad, dyad, triad, etc.