Derived from the Ancient Egyptian word psD.t, Ennead refers to a grouping of typically 9 gods. The earliest and most important Ennead was the one of Heliopolis, the center of solar cult in Ancient Egypt. It consisted of the primeval god Atum (solar god), his two children Shu (air) and Tefnut (moisture), their offspring Geb (earth) and Nut (sky) and their children Osiris, Isis, Seth and Nephthys. Horus and Thot are sometimes also counted to this Ennead, which makes the actual number of gods greater than 9.
 

Seven of nine members of the Heliopolitan Ennead, as represented in the judgement scene on the Papyrus of Ani. From right to left: Atum, Shu, Tefnut, Geb, Nut and sitting next to each other, Isis and Nephthys.

An Ennead could indeed contain more or less than 9 gods as the number 9 can be interpreted as the plural of a plural and a plural would typically consist of 3. Thus the Theban Ennead consisted of as many as 15 gods, whereas the Ennead of Abydos only counted 7. It is surprising to find that Amun, Mut and Khonsu, the primary triad of Thebes, are not usually part of the Theban Ennead, or that Osiris is not part of the Abydene Ennead.