Plexus
A plexus (from the Latin for "braid") is a branching network of vessels or nerves. The vessels may be blood vessels (veins, capillaries) or lymphatic vessels. The nerves are typically axons outside the central nervous system.
Although many medical words ending in -us that came to English from Latin have the plural suffix -i (and the plural form plexi indeed does exist in Latin), English does not use the -us/-i pattern for this particular term; the standard plural form in English is plexuses.