What is the section of the fuselage below the wings called?
The outer covers at-least, are called wing to body fairings in this Airbus document- 'Airbus Future Composite Wing'.



Wing to body fairings; image from Airbus Future Composite Wing

Elsewhere, the covers are called belly fairings by the manufacturers themselves, and also in the US patent US 20060065784 A1- Aircraft provided with a belly fairing, and corresponding belly fairing.

Anyway, the fairings enclose the center wing box and the main landing gear wheel well.