humility / reverence / awe (in worship)
Masculine verbal noun (maṣdar) of خَشَعَ (to be humble/reverent). Related: خَاشِعٌ (humble/reverent), خُشُوعٌ (humility). Differs from خَوْفٌ (fear) - khushūʿ combines awe, love, and humility.
Khushūʿ is considered the soul of prayer. Without inner humility and focus, prayer becomes mere physical movements. It is the first quality of successful believers mentioned in Surah Al-Muʾminūn.