I don't think that the auditor has any right to say terms and definitions need to be in the manual, but if a company wants to put them there, there is no problem with that either. The quality manual is there to present the big picture, help people understand what the company policies on quality management or business management systems are, and hopefully offer some value to the organization. So, simple answer is no, they are not required, real life answer is that it might add value in some industries, or depending on the overall knowledge of the intended audience.