تَسْمِيَةُ الشَّيْءِ بِاسْمِ جُزْئِهِ
Naming an entity by one of its parts.
Part-for-whole transfer occurs when an expression originally denotes a component but now signifies the entire entity. Not every incidental part makes a convincing label. The chosen component should usually be salient to recognizing the whole.
“ʿAyn” — literally an eye; in an appropriate context, a watcher or sentinel.
Watching is so closely associated with the eye that the part can designate the observer. The whole person, rather than merely the physical eye, is the intended referent.