the first 2 replies have define the greatness of Gurbani. which is agam agadh bodh.
but the question remains unanswered.
what is the DEFINATION of Gurbani?
Bani or Vani(in sanskrit) literally means "voice". Gurbani means "voice of the Guru"...though dass agrees there are many other arths of Gurbani.
Mohkam Singh ji, Guru Sahib also wrote numerous Hukamnamas to Sikhs in Kabul and other places. Still the panth doesn't considers them as Gurbani. It was also Bani(voice) which came from SatGuroo ji.