Orthostatic hypotension

Orthostatic hypotension (also known as postural hypotension, orthostatic intolerance and, colloquially, as head rush or a dizzy spell) is a sudden fall in blood pressure, typically greater than 20/10 mm Hg, that occurs when a person assumes a standing position, usually after a prolonged period of rest.
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Orthostatic hypotension is an abnormal decrease in blood pressure when a person stands up. [more]Orthostatic hypotension - Community and Resources

Noun
1. low blood pressure occurring in some people when they stand up
(synonym) postural hypotension
(hypernym) hypotension

A temporary low blood pressure (hypotension) due usually to suddenly standing up (orthostatic). Symptoms such as dizziness, feeling about to black out, and tunnel vision can be due to insufficient blood flow to the brain. The symptoms are typically worse when standing and improve with lying down. Orthostatic hypotension may be experienced by healthy people who rise quickly from a chair, especially after a meal, and have a few seconds of disorientation.