Lack of Oxygen. "ischemia" refers to any condition in which insufficient levels of oxygen are delivered to the tissues. Since oxygen is necessary for cell and tissue function, ischemia can cause symptoms, abnormal organ function and cell/tissue death. In the heart, for example, ischemia can result in chest pain and shortness of breath, loss of muscle strength and pump function and heart attack.
Low blood flow. Ischemia is basically low blood flow to an area of tissue, due to a decrease in oxygen and nutrients the tissue needs. That tissue can be anywhere in the body. When ischemia is severe in the heart it is called a heart attack, in the brain, a stroke. It can also occur in the kidneys and legs. If the circulation worsens, the ischemia can turn into gangrene.
Not enough blood. Ischemia is a condition where a part of the body does not get enough blood supply usually caused by a constriction or blockage of an artery. The red blood cells carry oxygen to the body so ischemia could also result if there is anemia (decrease in red blood cells) or a decrease in the amount of oxygen. It can lead to damage of the organ involved or to symptoms, chest pain if the heart is involved.