Buddho

He is called the Buddha for He was enlightened on eternal truths by himself and He enlightened others on that.

“BujjhitA saccAnIti buddho, bodhetA pajayati buddho.”

 

(PathisambhidAmagga, KN)

The Buddha explained Himself why He is called Buddha as “He is called Buddha for having enlightened He preaches dhamma for the enlightenment of others (buddho so bhagavA bodhAya dhammaM deseti).”