A plexus is a network. In biology it has two meanings.
In many animals the processes of neurons join together to form a plexus or nerve net. This is the characteristic form of nervous system in the coelenterates and persists with modifications in the flatworms. The nerves of the radially symmetrical echinoderms also take this form, where a plexus underlies the ectoderm of these animals and deeper in the body other nerve cells form plexuses of limited extent. In vertebrates nerves branch and rejoin in some parts of the body, for example the brachial plexus made up of the spinal nerves which enter the arm and the solar plexus above the stomach. Almost a hundred such plexuses have been named in the human body.
A plexus is also a network of blood vessels, with the choroid plexuses of the brain being the most commonly mentioned example. Choroid plexuses are very thin and vascular roof plates of the most anterior and most posterior cavities of the brain which expand into the interiors of the cavities. Other vascular plexuses are found elsewhere in the body.