The Viminal Hill (Latin Collis Viminalis, Italian Viminale) is the smallest and least important of the famous seven hills of Rome, and as such always referred to as collis rather than mons.
External link
Samuel Ball Platner, A Topographical Dictionary of Ancient Rome: (http://penelope.uchicago.edu/Thayer/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/.Texts/PLATOP*/Viminalis.html) Viminal Hill
The other six of the Seven Hills of later-Rome are the Aventine Hill (Collis Aventinus), the Capitoline Hill (Collis Capitolinus), the Quirinal Hill (Collis Quirinalis), the ViminalHill (Collis Viminalis), the Esquiline Hill (Collis Esquilinus), and the Caelian Hill (Collis Caelius).
The now-famous Vatican Hill (Collis Vaticanus) is northwest of the Tiber and is not one of the Seven Hills of Rome.
Likewise, the Pincian Hill (Pincius Mons), to the north, and the Janiculum (Ianiculum), to the west, are not counted among the traditional Seven Hills.
The other six of the Seven Hills of Rome are the Aventine Hill (Collis Aventinus), the Capitoline Hill (Collis Capitolinus), the Quirinal Hill (Collis Quirinalis), the ViminalHill (Collis Viminalis), the Esquiline Hill (Collis Esquilinus), and the Caelian Hill (Collis Caelius).
The now-famous Vatican Hill (Collis Vaticanus) is west of the Tiber and is not one of the Seven Hills of Rome.
The Seven Hills of Rome are now very different: five of them (in Italian) (Aventino, Celio, Esquilino, Quirinale, Viminale) are populated areas with monuments, buildings and parks; the Campidoglio (Capitol Hill) now hosts the Municipality of Rome; the Palatino is an archaeological area.