Wikipedia
A ribu is a mountain that reaches a topographic prominence of at least . The term "ribu" derives from the Indonesian word "ribu" meaning "thousand", and reflects that to qualify as a ribu a peak must reach a prominence of at least one thousand meters.
In Indonesia, three categories of ribus are known according to the absolute height of the peak. The "Sangat Tinggi" (Indonesian for "very high") category is for peaks higher than 3000 meters, "Tinggi Sedang" (Indonesian for "medium height") for peaks between 2000 and 3000 meters, and "Kurang Tinggi" (Indonesian for "less high") for peaks with an elevation of between 1000 and 2000 meters. Currently, a total of 233 ribus are known across the Indonesian archipelago. Some are popular hikes, such as Mount Rinjani, Mount Semeru, and Mount Kerinci, while others are much more obscure, and some do not even have official names.
Some famous Indonesian mountains, such as Mount Bromo and Tangkuban Perahu, are not ribus because they are connected to higher peaks by high passes and therefore do not achieve enough topographic prominence. However, a subsidiary category of spesial (Indonesian for "special") peaks contains those deemed of such significant touristic interest that they merit inclusion, albeit subjectively, in a secondary list. At the moment, the Gunung Bagging website counts 74 Indonesian spesials.
The list of the Indonesian ribus was compiled by Andy Dean and Daniel Patrick Quinn. As of September 2012, nobody is known to have completed the list.
While the term "ribu" has been adopted to describe "mountains that exceed a prominence of 1000 meters" also outside Indonesia, the "spesial"-category remains acknowledged only there.