A nakira is an indefinite noun — one that refers to any member of its kind rather than a specific, known one, like English “a book” or “a man.” Its usual sign is tanwin — the -un / -an / -in ending — together with the absence of the article الْ. Telling whether a noun is indefinite or definite is the first step in reading any Arabic phrase, because it decides how the noun behaves in the sentence.

From the free course The Language of Quran — Easier than English (Book 1) (LoQ1), taught by Ustad Muhammad Arjan Ali.