The triangular depression in the medial surface of each lung, where blood vessels and the primary bronchi enter, is the:

I believe the answer is hilum. Each lung can be divided into four main sections: the apex, base, root, and the hilus, or hilum of the lung. Hila or lung roots, are relatively complicated structures that consist mainly of the major bronchi and the pulmonary arteries and veins. The hilum of the lung is found on the medial aspect of each lung, and it is the only site of entrance or exit of structures associated with the lungs. That is both lungs have a region called hilum which serves as the point of attachment between the lung root and the lung.