Keroncong (various other spellings include Kronchong, Kroncong, etc.) is a popular music style which originated in the region now known as Malaysia and Indonesia. It was reputedly derived from local attempts to copy Portuguese popular songs brought to Malacca by traders and usually involved the use of stringed instruments. A local variant on the European lute resembling a ukulele was called the keroncong, so this may be how the style acquired its name.
Obvioulsy the style must have developed considerably over the time between the earliest European influences in the 16th century and modern times but there are no recorded examples before the early 20th century. By the time the earliest recordings were made (in Singapore in 1903 by The Gramophone Co. and in Batavia and Singapore in 1906 by Beka) keroncong was primarily a vocal style. Many similar recordings were made up until the early 1940s when all recording activity stopped due to the Japanese invasion of Southeast Asia. By this time accompaniments varied considerably and were not necessarily by the traditional group of stringed instruments (in some cases more modern instruments like the piano and saxophone were used). On most recordings the term keroncong applies to any popular vocal in the keroncong style whether stringed instruments were involved or not.
After WWII the recording of this style continued, but post-war recordings already show some new developments. While most pre-war keroncong records are played fast and sung in a high-pitched nasal voice, later recordings tend to be slower in tempo and the singing style is also different (becoming more similar to other forms of popular songs of that time). The keroncong style was still popular, but it began to lose it's dominant role in the local culture as other types of music gained more influence.
For post-war recordings if the performance is not specified as keroncong (or some variant of that term) this tag should not be used. Although recordings of keroncong music continue up to the present day, by the late 1970s it had been largely overtaken by other forms. In the 1970s & 1980s the dominant form in Indonesia was Dangdut. Although both Keroncong and Dangdut are well known forms of Indonesian popular music, they should be regarded as distinct styles and these tags would not normally be used together.