as it is used here in this verse, means,

Pure; simple; incorrrupt; unbiased; having clear vision of the divine truth.

Webster's 1828 American Dictionary of the English Language

The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light. But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness!
Matthew 6:22-23