An information exchange protocol used primarily in healthcare environments. It allows for a standard format and protocol for exchanging key sets of data among healthcare computer systems. The standard currently addresses the interfaces among various systems that send or receive patient admissions/registration, discharge or transfer (ADT) data, queries, orders, results, clinical observations, billing, and master file update information.

It is an ANSI standard.

Source:
http://www.mcis.duke.edu/standards/HL7/faq/HL7FAQ10.HTM#X4X1X1X