What does choroid mean?
The choroid is the middle layer of the eye's wall, sandwiched between the light-sensitive retina on the inside and the tough white sclera on the outside. It is densely packed with blood vessels and pigmented cells, giving it a dark colour; the vessels deliver oxygen and nutrients to the outer layers of the retina, while the pigment absorbs excess light that would otherwise blur vision by reflecting around inside the eyeball. In anatomy and medicine the term appears most often in phrases such as 'choroid plexus', a structure within the brain's ventricles that produces cerebrospinal fluid, and in conditions like choroidal neovascularization, a cause of vision loss in macular degeneration. Derived chiefly from Greek roots meaning 'membrane-like', the word belongs firmly to formal scientific vocabulary rather than everyday speech, yet it names one of the most biologically active tissues in the human body, sustaining the very cells that make sight possible.
nounThe pigmented, richly blood-vesselled middle layer of the eyeball, lying between the retina and the sclera, which nourishes the outer layers of the retina.
- The vascular, pigmented middle coat of the eyeball between the retina and the sclera, responsible for nourishing the outer retinal layers and absorbing stray light.
"Damage to the choroid can deprive the retina of vital nourishment."
"The ophthalmologist noted thickening of the choroid on the ultrasound scan."
"Because the choroid supplies blood to the photoreceptors, disorders affecting it often impair vision first."
Rarely used in the plural except when comparing structures across species or both eyes in comparative anatomy.
"The study compared the thickness of the choroids in birds and mammals."
The word choroid literally means 'like a membrane' in Greek — but this humble-sounding layer carries more blood flow per gram than almost any other tissue in your body.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of choroid
The word 'choroid' derives from the ancient Greek 'khorioeidēs', formed from 'khórion' (a membrane or afterbirth) combined with '-eides' ('in the form of'), so it literally means 'membrane-like'. It entered English in the early modern period through New Latin anatomical terminology as scholars translated classical Greek medical texts. The same Greek root gives us 'chorion', the embryonic membrane, and the related combining form '-oid' ('resembling') recurs throughout scientific English in words such as 'asteroid' and 'humanoid'.
Related word forms
How choroid is actually used
Strictly anatomical and medical in register; rarely encountered outside clinical, ophthalmic, or academic contexts. The derived adjective 'choroidal' is especially common in medical writing.
Easily confused with choroid
The chorion is the outermost membrane surrounding an embryo, whereas the choroid is a vascular layer within the eye.
The cornea is the transparent front surface of the eye that admits light, while the choroid is a dark, blood-vessel-rich layer at the back of the eye.