A commissure is the place where two things are joined. The term is used especially in the fields of anatomy and biology.
In anatomy, commissure refers to a bundle of nerve fibers that cross the midline at their level of origin or entry (as opposed to a decussation of fibers that cross obliquely).The most common usage of the term refers to the brain's commissures, of which there are five—the anterior commissure, posterior commissure, corpus callosum, hippocampal commissure (commissure of fornix), and habenular commissure—and which consist of fibre tracts that connect the two cerebral hemispheres and span the longitudinal fissure. In the spinal cord is found the anterior white commissure.
Commissure may also refer to cardiac anatomy of heart valves. In the heart, a commissure is the area where two valve leaflets have abnormally come together.
The term may also refer to the junction of the upper and lower lips (see labial commissure of mouth).
It may refer to the junction of the upper and lower mandibles of a bird's beak, or alternately, to the full-length apposition of the closed mandibles, from the corners of the mouth to the tip of the beak.
It may refer to the upper and lower eyelids.
In female genitalia, the joining points of the two folds of the labia majora create two commissures - the anterior commissure just anterior to the prepuce of the clitoris, and the posterior commissure of the labia majora, directly posterior to the frenulum of the labia minora and anterior to the perineal raphe.
In biology, the meeting of the two valves of a brachiopod or clam is a commissure; in botany, the term is used to denote the place where a fern's laterally expanded vein endings come together in a continuous marginal sorus.