pentad

plural pentads

1. Synonym of quintet: A group of five things, particularly
   1. Synonym of quinquennium: a five-year period, especially in reference to the first and second halves of calendrical decades.

      Synonyms: quinquennium

   2. climatologyA five-day period, especially a temperature average taken every five days.
   3. A five-day division of the Chinese calendar, forming part of a solar term (Chinese: 候).
   4. chemistryA pentavalent element or group: an atom or molecule with a valence of five, comparable with five atoms of hydrogen.

      Nitrogen is a pentad in the ammonium compounds.