The dorsum of a structure is the back of that structure.

Because limbs can rotate and joints can be bent, anatomists define the term with the human body standing with the hands out-turned, the dorsum of the hand is the back of the hand. The dorsum of the foot is the top of the foot.

This slideshare may help with anatomical descriptive terms -

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Dr Sheila StroverĀ (Editor)
BSc (Hons), MB BCh, MBA

See biography...