noun

• (countable, anatomy) The outer layer of an internal organ or body structure, such as the kidney or the brain.
  "In the study, led by Ryota Kanai of the University College London, people who identified themselves as liberals generally had a larger anterior cingulate cortex — a comma-shaped region near the front of the brain that is involved in decision-making. By contrast, those who identified as conservatives had larger amygdalas — almond-shaped structures that are linked with emotional learning and the processing of fear."
• (uncountable, botany) The tissue of a stem or root that lies inward from the epidermis, but exterior to the vascular tissue.
• (archaeology) The outer surface of a piece of flint.