This is the fifth in a series of essays on terms used in solar-terrestrial physics that are thought to be in need of clarification. Terms are identified and essays are commissioned by a committee chartered by Division II [Sun and Heliosphere) of the International Astronomical Union. Terminology Committee members include Ed Cliver (chair), Jean-Louis Bougeret, Hilary Cane, Takeo Kosugi, Sara Martin, Rainer Schwenn, and Lidia van Driel-Gestelyi. Authors are asked to review the origins of terms and their current usage/misusage. The goals are to inform the community and to promote discussion.
The following article by Chris St. Cyr and colleagues focuses on the term “halo coronal mass ejection (CME).” Halo CMEs are of great interest and import for space weather, but as St. Cyr et al. point out, their definition encompasses various forms of solar eruptive activity, not all of which may affect environmental conditions near Earth. St. Cyr et al. go further and list practical steps being taken to prevent confusion over the use of the term “halo CME” in the future. The community is encouraged to send feedback on this article to the authors, and suggestions for other terms to address in this forum to the IAU Committee.