The sublime is the quality of greatness, whether physical, moral, intellectual, metaphysical, aesthetic, spiritual, or artistic. The term especially refers to a greatness beyond all possibility of calculation, measurement, or imitation.
In Romanticism: Edmund Burke’s Philosophical Enquiry (1757) connected the sublime with experiences of awe, terror and danger. Burke saw nature as the most sublime object, capable of generating the strongest sensations in its beholders. This Romantic conception of the sublime proved influential for several generations of artists.
Definition adapted from Wikipedia and the Tate.