I think it is decreased blood supply to organs and tissues. I also think it is the stopped circulation. However, both ones cannot be right, I think.
What is ischaemia exactly?
Ischaemia is lack of sufficient perfusion to a tissue. Reduced perfusion could therefore mean decreased blood supply to organs and tissues and could also mean a complete cessation of blood flow to an area - therefore both can be correct as they are on a continuum. Infarction is tissue death due to hypoxia which can result from ischaemia that is not swiftly resolved.