A commissure is the place where two things are joined. The term is used especially in the fields of anatomy and biology.

In anatomy

Anatomy

Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant 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

Decussation

Decussation is used in biological contexts to describe a crossing.Examples include:* In the brain, where nerve fibers obliquely cross from one lateral part to the other, that is to say they cross at a level other than their origin...

 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
  Anterior commissure
  The anterior commissure is a bundle of nerve fibers , connecting the two cerebral hemispheres across the midline, and placed in front of the columns of the fornix...
  
  , posterior commissure
  Posterior commissure
  The posterior commissure is a rounded band of white fibers crossing the middle line on the dorsal aspect of the upper end of the cerebral aqueduct. It is important in the bilateral pupillary light reflex....
  
  , corpus callosum
  Corpus callosum
  The corpus callosum , also known as the colossal commissure, is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication...
  
  , hippocampal commissure (commissure of fornix), and habenular commissure
  Habenular commissure
  The habenular commissure, is a brain commissure situated in front of the pineal gland that connects the habenular nuclei on both sides of the diencephalon....
  
  —and which consist of fibre tracts that connect the two cerebral hemisphere
  Cerebral hemisphere
  A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...
  
  s and span the longitudinal fissure. In the spinal cord is found the anterior white commissure
  Anterior white commissure
  The anterior white commissure, also known as the alba anterior medullae spinalis, is a bundle of nerve fibers which cross the midline of the spinal cord just anterior to the gray 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 lip
  Lip
  Lips are a visible body part at the mouth of humans and many animals. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech...
  
  s (see labial commissure of mouth).
• It may refer to the junction of the upper and lower mandibles of a bird's beak
  Beak
  The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...
  
  , 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 eyelid
  Eyelid
  An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...
  
  s.
• In female genitalia, the joining points of the two folds of the labia major create two commissures - the anterior commissure just anterior to the prepuse of the clitoris, and the posterior commissure of the labia majora, directly posterior to the fenulum of the labia minora and anterior to the perineal raphe.

In biology

Biology

Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, the meeting of the two valves of a brachiopod

Brachiopod

Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

 or clam

Clam

The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

 is a commissure; in botany

Botany

Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...

, the term is used to denote the place where a fern

Fern

A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...

's laterally expanded vein endings come together in a continuous marginal sorus

Sorus

A sorus is a cluster of sporangia .In fungi and lichens, the sorus is surrounded by an external layer. In some red algae it may take the form of a depression into the thallus....

.