A term used to describe inflammation of the eye that causes the eye to have a red appearance. Depending on its cause, red eye may be accompanied by pain, tearing, tenderness, or changes in vision. Since red eye can have a variety of causes, the eye needs to be examined by a medical professional to determine the underlying disease. Possible causes include infection in the membrane covering the eye and eyelid (conjunctivitis), inflammation and infection in the eyelid (stye, chalazion, or blepharitis), infection of the tear sac (dacryocystitis), injury (eye injury), a foreign body, glaucoma, allergy, environmental irritants, or inflammation of the eyeball.