commissure

Imagine the corners of your mouth where the upper and lower lips meet—that is a commissure. It refers to a place where two parts join, often forming a clear line or seam. You most often see it in anatomy, such as the corners of your lips or the corners of your eyelids, but it also names important connections in the brain. In neurology, a commissure is a bundle of nerve fibers that links the left and right halves, allowing them to share information.

The word comes from the Latin commissura, meaning “a joining together.” Think of it as a small bridge between two sides.

A commissure is a point where two things join; it is a junction, seam, or line of connection. While the term sounds technical, you encounter commissures every time you look in the mirror. The most common biological examples are the oral commissures (the corners of the mouth) and the palpebral commissures (the corners of the eyes). These are specific points where distinct parts of the body meet and fuse.

Beyond external anatomy, “commissure” is a crucial term in neurology. It describes bundles of nerve fibers—such as the corpus callosum, as well as the anterior and posterior commissures—that cross the midline of the brain to connect the left and right hemispheres. In this sense, a commissure is not just a meeting point, but a working pathway for communication. In botany, the term can also be used for the joining surface where two parts of a fruit are held together.

While predominantly used in medical and scientific writing, it can be borrowed in a broader way to mean any spot where two separate streams meet—where rivers join, where communities connect, or where ideas touch and start to blend. Whether you are discussing the delicate anatomy of the human face, the complex wiring of the brain, or a more figurative meeting point, a commissure is fundamentally the exact place where two separate things become one.