Hep"tad (?), n. [L. heptas the number seven. Gr. &?;, &?;, fr. "epta` seven.] (Chem.) An atom which has a valence of seven, and which can be theoretically combined with, substituted for, or replaced by, seven monad atoms or radicals; as, iodine is a heptad in iodic acid. Also used as an adjective.