وَهُوَ عِنْدَ النُّحَاةِ: مَا لَا يَكُونُ فِي آخِرِهِ حَرْفُ عِلَّةٍ، كَـ«زَيْدٍ».
Translation: According to grammarians, a sound-final noun is one whose final letter is not a weak letter, such as “زَيْدٍ.”
زَيْد is a singular noun ending in the consonant د rather than a weak letter. It therefore illustrates this class.
Here “sound” is a technical description of the final letter, not a judgment that other Arabic words are incorrect.