さんぷく山腹sanpuku
1.
The area of a mountain situated between the peak and the base, often characterized by slopes and varied terrain that can be steep or gentle.
2.
The section of a mountain that is not at the top or the bottom, typically where vegetation may thrive and natural features are prominent.
3.
A mid-elevation part of a mountain, which can be considered a transition zone between the summit and the foothills.
Example:
僕らの大学の校舎は、今福岡市の北部の山腹に建築中です。