Fear of the dark: Pathological fear of the dark, an abnormal and persistent dread of the dark. Sufferers experience undue anxiety even though they may rationally realize that the dark does not pose a threat commensurate with their fear.
Fear of the dark is termed "nyctophobia," a word derived from the Greek "nyx" (night) and the Greek "phobos" (fear). "Nyx" has also contributed to other medical terms such as "nyctalopia" (night blindness) and "nyctanopia" (impaired vision in dim light and in the dark). Other terms for fear of the dark include scotophobia and achluophobia.