This title as first given to the third Dalai Lama, Sonam Gyatso, by Altan Khan. In order to help secure his rule in Mongolia, Altan Khan agreed to let Sonam Gyatso convert Mongolia to Buddhism.
Gyatso then proclaimed that Altan Khan was in fact the reincarnated
of the great Khublai Khan, former ruler of China and Mongolia and
grandson of Ghengis Khan, helping legitimize Altan Khan’s rule. The
fourth Dalai Lama, Yonten Gyatso, the only Dalai Lama to be born
outside of Tibet to date was the great grandson of Altan Khan.
The title Dalai Lama literally means 'The Ocean Lama' with the
'lama' coming from the Tibetan 'blama', meaning guru, mentor,
teacher, wise-one.” So, it could be interpreted as 'Ocean of