heptad vs seven

heptad

noun

  • A sequence of seven bases. 

  • Synonym of septet: a group of seven things. 

seven

noun

  • The digit/figure 7 or an occurrence thereof. 

  • A card bearing seven pips. 

num

  • A numerical value equal to 7; the number following six and preceding eight. This many dots: (•••••••). Describing a group or set with seven elements. 

How often have the words heptad and seven occurred in a corpus of books? (source: Google Ngram Viewer )