What is ventral and dorsal side?

Dorsal and ventral describe opposite body surfaces used for anatomical orientation. In animals: dorsal = the back side; ventral = the belly side. For a fish, the dorsal fin is on top; the ventral surface faces the substrate. In humans standing upright, dorsal often corresponds to the posterior of the trunk, while ventral corresponds to the anterior (front) of the trunk; in limbs and head, usage follows standard anatomical planes. These terms are independent of left/right and help locate organs, injuries, or surgical approaches precisely.

In plants, especially leaves, surfaces are also differentiated. A typical dorsiventral leaf (e.g., dicots) has a distinct adaxial (upper, dorsal) surface that is usually greener, thicker, and receives more light, and an abaxial (lower, ventral) surface that bears more stomata for gas exchange. 

The distinction reflects functional polarity: light capture and protection above, and transpiration and gas exchange below. In stems or organs with radial symmetry, dorsal–ventral polarity may be absent.