Ischemia describes any restriction in the blood supply to any part of the body. By comparison, a complete blockage is an infarction. It is generally caused by a constriction of the blood vessels, clotting, or an embolism. It almost always presents with cyanosis. An ischemia generally causes a shortage of oxygen in the affected tissues, rather that a total absence of oxygen. It can also cause a lack of glucose or other blood-borne nutrients to tissues as well.
A patient exhibiting ischemia and cyanosis, courtesy James Heilman MD, via Wikipedia
Some examples of ischemia are angina, caused by insufficient blood flow to the heart, and a transient ischemic attack, caused by lack of blood flow to part of the brain.
Many conditions can cause ischemia, such as atherosclerosis, hypoglycemia, tachycardia, low blood pressure, clots, tumors pressing against a blood vessel, sickle cell anemia, extreme cold, and arteriovenous malformation.
An untreated ischemia can lead to tissue necrosis. In oxygen dependent organs such as the heart and brain, damage can occur in as little as three minutes, but after twenty minutes, the damage is usually irreversible. In many cases, restoration of blood flow can do more damage, leading the release of free radicals which result in reperfusion injury.