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. 2019

Examples from the Web for heptad

Historical Examples of heptad

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


    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



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