An Etymological Dictionary of Astronomy and Astrophysics

1. A group of four.
   

2. A sequence of four → total lunar eclipses occurring at approximately six month intervals. Tetrads are relatively rare events. The first → total eclipse of a tetrad occurred on 15 April 2014, the next eclipse of the series will happen on 8 October 2014 and the next two eclipses on 4 April and 28 September 2015, respectively. The previous tetrad happened in 2003-2004 and the next one will be in 2032-2033.

Etymology (EN): From Gk. tetrad-, stem of tetras “group of four.”

Etymology (PE): Cahârgân, from cahâr, → four, + -gân suffix of plurality.