Viable brain tissue around the irreversibly damaged ischemic core after a stroke. In other words, it is a region of the brain that has lost only some of its blood supply, and retains structural integrity but has lost function (i.e., the tissue that was almost turned dark by an ischemic attack). Its medical connotation can be better understood when understanding that, figuratively, penumbra refers to an area of uncertainty or intermediacy between two mutually exclusive states or categories.