What is a nominal sentence (jumlah ismiyyah)?
A sentence that opens with a noun is a جُمْلَة اِسْمِيَّة (jumlah ismiyyah). It has two parts and no verb is needed.
Every jumlah ismiyyah has a mubtada' — what it is about — and a khabar — what is said about it. Both are marfū'.
In مُحَمَّدٌ مُجْتَهِدٌ (muhammadun mujtahidun), “Muhammad is hard-working”, مُحَمَّد is the mubtada' and مُجْتَهِد is the khabar. English needs is; Arabic does not.
The order can turn over. When the khabar comes first it is مُقَدَّم (muqaddam), and the mubtada' behind it is مُؤَخَّر (mu'akhkhar).
Like most Arabic grammar terms, اِسْمِيَّة (ismiyyah) was taken from everyday speech and then given one precise technical sense.
In everyday Arabic it simply means “to do with a name”.
جُمْلَةٌ اِسْمِيَّةٌ (jumlatun ismiyyatun) — “a noun-sentence”
In Arabic grammar it marks the sentence that opens with a noun rather than a verb.