[ hep-tad ]
/ ˈhɛp tæd /


the number seven.
a group of seven.
Chemistry. an element, atom, or group having a valence of seven.

Origin of heptad

First recorded in 1650–60, heptad is from the Greek word heptad- (stem of heptás). See hepta-, -ad1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

Examples from the Web for heptad

  • But this oddness belongs to other numbers also (pentad, heptad, &c.).

    Aristotle|George Grote
  • Heptad, a term in chemistry to denote an atom that is the equivalent of seven atoms of hydrogen, from hepta, seven.

    The Nuttall Encyclopaedia|Edited by Rev. James Wood

British Dictionary definitions for heptad

/ (ˈhɛptæd) /


a group or series of seven
the number or sum of seven
an atom or element with a valency of seven

Word Origin for heptad

C17: from Greek heptas seven
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012