The Nilgiri Mountains form part of the Western Ghats in northwestern Tamil Nadu, Southern Karnataka, and eastern Kerala in India. They are located at the trijunction of three states and connect the Western Ghats with the Eastern Ghats. At least 24 of the Nilgiri Mountains' peaks are above , the highest peak being Doddabetta, at .
The word Nilgiri, comes from Sanskrit words neelam (blue) + giri (mountain), has been in use since at least 1117 CE. In Tamil literature it is mentioned as Iraniyamuttam It is thought that the bluish flowers of kurinji shrubs gave rise to the name.
The Nilgiri Hills are separated from the Karnataka Plateau to the north by the Moyar River.
Three national parks border portions of the Nilgiri mountains. Mudumalai National Park lies in the northern part of the range where Kerala, Karnataka, and Tamil Nadu meet, covering an area of . Mukurthi National Park lies in the southwest part of the range, in Kerala, covering an area of , which includes intact shola-grassland mosaic, habitat for the Nilgiri tahr. Silent Valley National Park lies just to the south and contiguous with those two parks, covering an area of .
The Nilgiri Hills are part of the Nilgiri Biosphere Reserve (itself part of the UNESCO World Network of Biosphere Reserves.), and form a part of the protected bio-reserves in India.
The high steppes of the Nilgiri Hills have been inhabited since prehistoric times, demonstrated by a large number of artifacts unearthed by excavators. A particularly important collection from the region can be seen in the British Museum, including those assembled by colonial officers James Wilkinson Breeks, Major M. J. Walhouse and Sir Walter Elliot.
The first recorded use of the word Nila applied to this region can be traced back to 1117 CE. In the report of a general of Vishnuvardhana, King of Hoysalas, who in reference to his enemies, claimed to have "frightened the Todas, driven the Kongas underground, slaughtered the Poluvas, put to death the Maleyalas, terrified Chieftain Kala Nirpala and then proceeded to offer the peak of Nila Mountain.