An ISO loom is a type of wiring loom (also called a "wiring harness") and connector that is used in car stereos, radios and satellite navigation systems. If you install an aftermarket stereo or similar device in a car without an ISO connector, you will typically need an ISO loom to adapt the new device.
ISO, short for International Organization for Standardization, is the organisation that, in 1992, set the industry standard for car radio/stereo connections. In car audio terminology, the actual standard, ISO 10487, is commonly shortened to just ISO.
An ISO connector includes at least two--and up to four--sets of pin connectors, typically labelled 'A,' 'B,' 'C' and 'D.' The required sets (A and B) control simple functions, such as power and speaker strength, while the optional sets (C and D) are only used for advanced functions, such as GPS control.
Most older car models (pre-1992) and several newer car models, including the Jaguar S-Type and Mercedes-Benz E-Class, do not come with industry-standard ISO 10487 connectors. If installing an aftermarket stereo or similar device in these cars, you will need an ISO loom adaptor.
Some car stereos and related devices, including various Kenwood stereos and Parrot GPS devices, also require special ISO looms to adapt to most car models.