The Greek adjective haplous means 'single' or 'simple' in the sense of having one fold — undivided, without duplicity. Jesus uses it of the eye, describing a condition of spiritual clarity and undivided focus.
In Matthew 6:22 (parallel Luke 11:34), Jesus delivers one of his most enigmatic sayings: 'The eye is the lamp of the body. If your eye is haplous ('healthy'/single), your whole body will be full of light.' Haplous here likely carries both the physical sense of a healthy eye that sees clearly and the moral sense of a heart without divided loyalties — focused on one Master (God, not mammon). The haplous eye is the eye undistorted by greed and undivided by competing loves. It perceives reality as God does: clearly, simply, without the distortion that double-mindedness introduces.