The choroid is the layer of blood vessels and connective tissue between the white of the eye and retina (at the back of the eye). It is part of the uvea and supplies nutrients to the inner parts of the eye. Inflammation of the choroid is called choroiditis.
Regatieri CVS, Roh S, Weiter JJ. Retinal and choroidal circulation. In: Yanoff M, Duker JS, eds. Ophthalmology. 5th ed. Philadelphia, PA: Elsevier; 2014:chap 6.3.