عَيْنٌ

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – عَيْنٌ is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

عَيْنٌ is the natural state of this indefinite noun and it is in this state ( مَرْفُوْعٌ) when the noun is: 1) the subject المُبْتَدَأُ or 2) the predicate الخَبَرُ or 3) اسْمُ كَانَ or 4) خَبَرُ إِنَّ or 5) الفَاعِلُ subject of a verb in the active voice or 6) نَائِبُ الفَاعِلِ subject of a verb in the passive voice or 7) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

Eye; Look; Hole; But of a tree; Spy; Middle letter of a trilateral word; Spring of water; Chief; Personage of a place

To hurt in the eye, smite anyone with the evil eye, flow tears, become a spy

Surah/Below is the only verse in the holy Quran containing the indefinite nounعَيْنٌ
Verse 
88-12therein will be a running springفِيهَا عَيْنٌ جَارِيَةٌ