What is enlightenment?

Buddhist Monk and Peace Activist, Thich Nhat Hanh (who passed away several years ago in his nineties) explained enlightenment succinctly and beautifully - Enlightenment is when a wave realizes it is the ocean. - Thich Nhat Hanh

Rumi, 13th Century Poet and Sufi Islamic Scholar, also explained this concept elegantly - 'You are not a drop in the ocean. You are the entire ocean in a drop.'