Noun
1. A sensory structure capable of light reception, appearing as a spot on the top of the head in lampreys, hagfish, amphibians, and some reptiles, especially the tuatara. Pineal eyes can contain a rudimentary cornea, lens, cone cells, and retina, and are thought to be sensitive to light and dark but not to be able to form images. (The American Heritage® Science Dictionary Copyright © 2005 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.)
2. A sensory structure capable of light reception located on the dorsal side of the diencephalon in various reptiles. (wordnetweb.princeton.edu)
3. An outgrowth of the pineal gland that forms an eyelike structure on the top of the head in certain cold-blooded vertebrates. (Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003)
4. A gland-like structure in the brain which appears to be the major site of melatonin biosynthesis. The exact role of the pineal remain obscure. (biology-online.org)
Syn: Third eye
Word origin: From French pinéal, literally “like a pine cone,” from Latin pinea “pine cone,” from pinus “pine tree”