الوَتْرِ

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

الوَتْرُ is the natural state of this definite noun and it changes to الوَتْرِ due to: 1) a preposition that precedes it or 2) a possessed noun that precedes it or 3) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

مَعْرِفَةٌمُفْرَدٌمُذَكَّرٌمَجْرُوْرٌو ت ربَاب ضَرَبَوَتَرَ يَتِرُفَعْلٌاسْمٌاِسْمُ الجِنْسِ
DefiniteSingularMasculineGenitiveRootOddضPatternNounGeneric or common Noun
Surah/Below is the only verse in the holy Quran containing the nounالوَتْرِ
Veres 
89-3and by the even and the oddوَالشَّفْعِ وَالوَتْرِ