LOINC is a rich catalog of measurements, including laboratory tests, clinical measures like vital signs and anthropomorphic measures, standardized survey instruments, and more. LOINC enables the exchange and aggregation of clinical results for care delivery, outcomes management, and research by providing a set of universal codes and structured names to unambiguously identify things you can measure or observe.
Put another way, LOINC provides the lingua franca for interoperable data exchange.
Today, it contains concepts for everything from a serum alpha 1 antitrypsin level to a zygomatic arch x-ray report. For each concept, LOINC contains many other rich details, such as synonyms, units of measure, and carefully crafted descriptions.