collis

Collis, collis, m., third declension i-stem: ablative colle, genitive plural collium. It is the small brother of mons. A collis is a rise you walk up; a mons is a mountain, and Latin holds that line rather more firmly than English holds the one between hill and mountain.

Rome's own seven are the septem colles collectively, though several of them are individually called mons - mons Palatinus, mons Caelius - so Roman usage was never tidy about its own capital. The root is the same as in excello, "stand out", and celsus, "lofty", and English hill is the Germanic cousin of it rather than a borrowing. Its natural opposite in this corpus is vallis, the valley between.