The Sanskrit word for enlightenment is "bodhi," which means "awakened." But awakened to what?
The only true answer to the question is to realize enlightenment. Short of that, we must come up with provisional answers that, the teachers tell us, do not really do justice to enlightenment.
Enlightenment can be defined as the cessation of dukkha, which is another word usually mangled in translation. It can be defined as the full realization of the truth of the Buddha's teachings. It can be defined as awakening to a great reality most of us never perceive.
Enlightenment in the Buddhist sense has nothing to do with knowledge or intellect. Nor is it living in an otherworldly state of being "blissed out" or having visions or supernatural experiences.
In his book Zen Mind, Beginner's Mind, Shunryu Suzuki Roshi said that enlightenment is "nothing special. ... You may say 'universal nature' or 'Buddha nature' or 'enlightenment.' You may call it by many names, but for the person who has it, it is nothing, and it is something."