Concise Bible Dictionary:

Used symbolically for the omnipresence of God. “The eyes of the Lord are in every place” (Prov. 15:3); “the eyes of the Lord are upon the righteous” (Psa. 34:15; 1 Peter 3:12: Compare 2 Chron. 16:9; Zech. 4:10). His eyes are also upon the wicked, and His eyes will not spare, neither will He have compassion in the day of judgment (Ezek. 5:11). The eye is also used symbolically for the organ that transmits the light to the soul. If the eye is single—there being but one object (the glory of God) before the soul—the whole body is full of light; but if the eye be evil, having divers objects (as when an eye sees double), the whole body is full of darkness. And if the light (true light it may be) be darkness, how great is that darkness! A Christian in this condition may do the very things he had strongly condemned in others (Matt. 6:22-23; Luke 11:34-36).