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.
From Gk. tetrad-, stem of tetras "group of four."
Cahârgân, from cahâr, → four, + -gân
suffix of plurality.