الرَّابِعُ: الْمُنَادَى، وَهُوَ الْمَطْلُوبُ إِقْبَالُهُ، وَهُوَ اسْمٌ مَدْعُوٌّ بِحَرْفِ النِّدَاءِ لَفْظًا، نَحْوُ: «يَا عَبْدَ اللَّهِ».
Translation: The fourth is the vocative: a noun addressed or called to attend, as in “Yā ʿAbda llāhi”.
Munādā means the addressee of a call. The source introduces it here among constructions with an implied verb; its detailed categories follow.