Hypotension is defined as low pressure in the arteries of the systemic circulation.
Systolic blood pressure less than 90 mm Hg and diastolic pressure less than 60 mm Hg consider as low blood pressure.
Types:

• Orthostatic hypotension
• Neurally mediated hypotension
• Severe hypotension related to shock
• Postprandial hypotension

Causes:

• Dehydration
• Hormonal problems
• Low blood volume
• Anemia 
• Pregnancy
• Arrhythmic heartbeats
• Widening of blood vessels
• Side effects of certain medications
• Severe allergic reaction 
• Sepsis
• High body temperature
• Bleeding in excess

 Symptoms:

• Dizziness
• Nausea and vomiting
• Fatigue
• Cold and clammy extremities
• Weakness
• Lack of concentration
• Blurred vision
• Fainting 
• Rapid shallow breathing
• Rapid pulse

Treatment Include:

• Medicine 
• Consumption of fluids and water

Prevention:

• Eating small meals frequently
• Stay hydrated
• Get up slowly from sitting or standing position
• Slightly raise the head side of your bed
• Exercise regularly to promote blood flow
• Increase salt intake (but after the recommendation of doctor)
• Avoid alcoholic drinks