المِرْصَادِ

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

المِرْصَادُ is the natural state of this definite noun and it changes to المِرْصَادِ 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

Ambush (from where one watches the doing of the wicked)

To watch, lay in wait, observe

الْمَعْرِفَةُمُفْرَدٌمُذَكَّرٌمَجْرُوْرٌر ص دبَاب نَصَرَمِفْعَالٌرَصَدَ يَرْصُدُاسْمٌالْاِسْمُ الْآلَة
DefiniteSingularMasculineGenitiveRootWatchfulPatternنNounUtilitarian noun
Surah/Below is the 1 verse in the holy Quran, containing the definite, genitive noun المِرْصَادِ
Verse 
89-14Indeed, your lord is surely Ever Watchfulإِنَّ رَبَّكَ لَبِالمِرْصَادِ