I believe nonameter is metered line with 9 strong stresses. If it were iambic nonameter, it would be a line with 9 strong stresses and 9 weak stresses, for a total of 18 syllables. In other cases, such as cretic, anapestic, and dactylic, it would be sufficient to say that there were 9 strong stresses and thus the line was in nonameter of whatever kind.