Wave Spectrum is the concept used to describe mathematically the distribution of wave energy (proportional to the square of the significant wave height) with wave period. Using this concept, a highly confused pattern of interfering waves can be divided into its constituent wave-forms. The results can be used as an aid to wave forecasting (encyclopedia.com)
In simpler words, wave spectrum is the distribution of wave energy as a function of frequency. It describes the total energy transmitted by a wave-field at a given time.
(Because of the Internet Provider Connection error, the post will be done on Monday 8th February)