A feeling you are "going to faint." Lightheadedness is medically distinct from dizziness, unsteadiness, and vertigo. See: Dizziness, Unsteadiness, and Vertigo.
Lightheadedness is a common and often unpleasant sensation of dizziness and/or feeling that one may be about to faint, which may be transient, recurrent, or occasionally chronic. In some cases the individual may feel as though his or her head is weightless. It can be simply an indication of a temporary shortage of blood or oxygen to the brain, low blood pressure, low blood sugar, or anemia.