The term ENNEAD MINUS ONE is a mathematical expression that refers to the difference between two numbers. The first number is an ennead, which is a number equal to nine, and the second number is one. The crossword solutions for this term are OCTET and OCTAD. An OCTET is a set of eight items, while an octad is a group of eight people or things. Therefore, the difference between an ennead and an OCTET is one, and the difference between an ennead and an octad is also one.