Caloocan as a toponym comes from the Tagalog root word lo-ok; kalook-lookan (or kaloob-looban) means "innermost area". The city's name is colloquially spelled as Kalookan.s
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Originally, Caloocan was the area where the old town of Tondo and Tambobong met, located along the shores of Dagat-Dagatan, a crescent-shaped inland lagoon to the west. The settlement along the shore was called “Aromahan”, or “Espina” to the Spaniards, and was separated from Manila Bay by a narrow ridge from Tondo towards an opening in Kinabutasanleading to the sea.
By the late 1700s, the fishermen of Aromahan has expanded towards a hill east of Dagat-Dagatan. This naturally stony hill was called “Kaloogan”, meaning “interior territory”, which evolved from the old Tagalog word “loog” (synonymous with “loob” or “inside”). The "g" sound could have shifted to the “k” sound in Tagalog phonetics (e.g. baksak > bagsak) leading to the present name of the city. With Aromahan relegated to the periphery, this hilltop area was also settled by oppressed people from Tondo, becoming the new center of the community by 1802. To the east was a vast stretch of cogon-covered land. Eventually called “Kalaanan”, meaning flat grassland in old Tagalog, this area is now generally known as Grace Park.
Unspecified location in Caloocan taken in the 1800's during the Spanish occupation.
Another 19th century photograph in libis Kalookan. Libis is located near the San Roque Cathedral.
Balintawak Beer Brewery located at the boarder of Caloocan and Quezon City this was from WWll imagery.
Caloocan is divided into two non-contiguous areas with a total combined area of 53.334 square kilometers. Southern Caloocan, with an area of 13.625 square kilometers, is bordered on the south by Manila, on the east by Quezon City, and on the north-northwest by Malabon, Navotas and Valenzuela. Northern Caloocan, with an area of 39.709 square kilometers, shares its border on the south-southeast by Quezon City, on the southwest by Valenzuela, and on the north by Marilao, Meycauayan and San Jose del Monte in the province of Bulacan.
As of 2015, the city has a population of 1,583,978 people, which makes it the fourth largest city in the Philippines in population. Under the same census year, Caloocan South (Barangays 1 to 164) has a population of 585,091 and Caloocan North (Barangays 165 to 188) has a population of 998,887. If the two districts are treated as separate cities, they will still be among the largest in country for the 2015 census year - ranking as the 4th and 17th with the highest population.
The population density of Caloocan (28,387 persons per square kilometer) surpasses that of the NCR population density. Of the country's 238 legislative districts (LDs), the First LD of Caloocan was the biggest in terms of population size, with 1.19 million persons. Most residents speak both Filipino and English, with considerable numbers speaking other languages and dialects.
Like many other places in the country, Roman Catholicism is the predominant religion. There is a significant presence of Iglesia ni Cristo and other Protestant churches like Church of God Caloocan located at Baesa, Caloocan.