We can not say anything aboutSAMADHI because whent nothing happen then Samadhi Happen. We can say only SUNYA. All thought will disappear.
State of Super bliss, joy and merging individual consciousness in to universal consciousness. Union between Jivatman and Paramatman. Union of Shiva and Shakti in Sahasrar Chakra (the top of the head). Realizing the Bramhan (pure consciousness) or Realization of God is the ultimate achievement of Human Birth.