In astronomy, a tetrad is a set of four total lunar eclipses within two consecutive years.^[1]

The frequency of tetrads varies by century with the frequency of total lunar eclipses. This repeating cycle is called a tetradia. See Eclipse cycle for more information.

• Total penumbral lunar eclipse § Summary frequency of total penumbral, total umbral and tetrad events 501–2500 for statistics by century
• Blood moon prophecy Apocalyptic preaching of John Hagee and Mark Biltz in part based on the observed phenomenon.