Concise Bible Dictionary:
1. ishoit. Gesenius says this word signifies “little man” and then “the little man of the eye”; that is, “the pupil of the eye in which, as in a mirror, a person sees his own image reflected in miniature.” He says “this pleasing image is found in several languages.” It is the part of the eye specially to be guarded: God preserved His own as the apple of His eye (Deut. 32:10; Psa. 17:8). His law should be kept as a precious thing (Prov. 7:2).
2. babah, the black or pupil of the eye, or, as others, “the gate of the eye.” To touch God’s people is touching the apple of His eye (Zech. 2:8).
3. bath, daughter. The sense is, Let not the apple (the daughter) of thine eye cease to shed tears (Lam. 2:18). In all places “the apple of the eye” is a beautifully figurative expression for that which must be tenderly cherished as a most choice treasure.