Retinal migraine

Retinal migraine (also known as ophthalmic migraines, or ocular migraines) is a medical condition that affects the eyes by causing vision loss in one eye for less than 1 hour, usually for about 10–20 minutes. When these retinal migraines happen they can also cause a headache, either a "diffuse" headache which is a headache that causes pain which is spread out and hurts a person's whole head, or, a "unilateral" headache, which is a headache that causes pain on one side of a persons head either, the left side or the right side.
It is believed retinal migraines are caused by "vasospasms "of the blood vessels of the eye in the area of the "retina". The retina is at the back of the inside of the eye. In medicine a "spasm" is a sudden constriction or tightening of muscles that is "involuntary" - a person does not control them they move on their own. Muscle spasms can happen in different muscles of the body, like when a person's leg or eye "twitches". The twitching is caused by the spasms.
"Vaso-" means something to do with the blood vessels, so a "vasospasm" causes the muscle cells inside the walls of a blood vessel to suddenly constrict or tighten up. This causes the whole blood vessel to "constrict" which means makes the blood vessel narrower. When a blood vessel constricts it is called "vasoconstriction".
When vasoconstriction happens less blood the normal is able to flow through the blood vessel, like the way squeezing (constricting) a garden hose makes less water come out. Vasoconstriction can happen for different reasons including because of a medical problem.
The second part of a vasospasm -"vasodilation" - occurs after the vasoconstriction. After the blood vessel constricts it "dilates" - becomes bigger. It may return to normal size or slightly larger than normal size.
The vasoconstriction and vasodilation complete one spasm. Spasms often happen many times over and over again. It is beleved that the muscle cells tigntening and loosenening may trigger the pain of a retinal migraine.