The opposite of ventral is dorsal. In anatomical terms, ventral means "towards the bottom side or towards the belly" of a living creature. Dorsal is the opposite of this and means "toward the top side or towards the back" of a living creature.
Both terms are used slightly differently by anatomists and zoologists. Anatomists, when speaking of the human body, use ventral to describe the front of the body and dorsal to describe the back. This is because humans are bipedal and usually are found upright. Zoologists, when speaking of four-legged animals, use ventral to describe the bottom of the animal and dorsal to describe the top.