The International Astronomical Union (IAU) is an organization which coordinates the work of national astronomy associations along with professional astronomers. The IAU promotes advancement in the science of astronomy, along with protection of astronomy as a scientific field. It is a member organization of the International Council for Science, a group of international scientific organizations which includes the International Union of Pure and Applied Chemistry and the International Mathematical Union, among many others.
This organization was founded in 1919 with the melding of several astronomy organizations. It is based in Paris, France, and includes individual professional members, most of whom are leaders in the field of astronomy, along with numerous national astronomy organizations. While the bulk of the International Astronomical Union's work is focused on advancements in professional astronomy, the organization also works with groups which include amateur astronomers among their members, and it recognizes the valuable contributions made by some amateur astronomers.
One of the key aspects of the work of the International Astronomical Union is the development of uniform and consistent terminology through the Working Group for Planetary System Nomenclature. The organization has extensively defined and designated terms for use in astronomy so that astronomers can clearly communicate with each other. When an astronomer writes about a “planet” in a scientific paper, for example, other astronomers know precisely what sort of celestial body that astronomer is referring to, thanks to the standards set by the IAU.
There are several parts to the IAU. The first is the General Assembly, a collective of the representative member nations which meets every three years to set policy. The locations of General Assembly meetings rotate, giving every member nation an opportunity to host the International Astronomical Union. The organization as a whole is broken into divisions, with each division subdivided into commissions. The commissions in turn have a large pool of working groups and programs which work on individual issues.
For reasons of standardization, astronomy texts usually stick with terminology which has been defined by the International Astronomical Union, and the organization publishes updates periodically so that people in the field can keep up with changing terminology. The definitions published by the IAU are not without controversy, however, as scientists are a notoriously stickling and argumentative bunch, and astronomers are no exception. The decision to downgrade Pluto from a "planet" to a "dwarf planet" and later a "plutoid," for example, attracted a great deal of attention.