Lightheadedness

Lightheadedness is a sensation of faintness, unsteadiness, or impending loss of consciousness, often without the spinning sensation characteristic of vertigo.

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Presyncope: A feeling that loss of consciousness is imminent, without complete loss of consciousness. It names the near-fainting form of lightheadedness.
Syncope: A brief loss of consciousness caused by transient global reduction in brain blood flow. Complete loss of consciousness distinguishes syncope from lightheadedness alone.
Dizziness: A broad term for sensations of altered balance, motion, or orientation. Lightheadedness is one distinct sensation commonly grouped under this umbrella term.
Postural orthostatic tachycardia syndrome: A chronic condition marked by excessive heart-rate increase on standing and orthostatic symptoms. It is one possible explanation for recurring posture-related lightheadedness.
Orthostatic hypotension: A sustained fall in blood pressure after standing or sitting upright. It can briefly reduce blood flow to the brain and cause faintness.
Vertigo: An illusion that the body or surroundings are moving or spinning. Its characteristic motion sensation differs from faintness, though symptoms can overlap.
Disequilibrium: A sense of instability or impaired balance while standing or walking. It captures the unsteadiness that can accompany lightheadedness.
Anemia: A reduced concentration of hemoglobin or red blood cells that limits oxygen transport. It can contribute to persistent fatigue and lightheadedness, but symptoms alone cannot establish it.
Cerebral hypoperfusion: Insufficient blood flow to the brain to meet its metabolic needs. Reduced cerebral blood flow can create a faint or woozy sensation.
Orthostatic vital signs: Blood pressure and heart rate measurements taken lying down and after standing. These measurements can reveal posture-related changes associated with lightheadedness.