Definition & Meaning of "the [apple] of {one's} (eye|eyes)"

What is the origin of the idiom "the apple of one's eye" and when to use it?

The origin of the idiom "apple of one's eye" can be traced back to ancient biblical and literary references. In the Bible, the phrase appears in various translations, referring to something or someone cherished or highly valued. It also has roots in ancient Greek and Latin literature, where the phrase was used metaphorically to describe something as precious and dear. It is used to express a deep affection or fondness for someone or something. It signifies that the person or object being referred to is highly cherished, loved, or regarded as the most precious in one's life. It conveys a sense of adoration, protectiveness, or special attention given to that person or thing.