noun

• 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.

• 3Physics
  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.