Dehydration (hypohydration) means less water in something. The word comes from the ancient Greek word for water, hydor.

People get dehydrated when they lose more water than they take in.  The human body needs water to survive.  If a person gets badly dehydrated, their body will be unable to work the right way.  Bad dehydration can cause organ damage and even death.

What causes dehydration? 
A living organism will get dehydrated if their body is losing water in some way and they do not drink enough to replace the water.  The most common causes of dehydration are: 
 Sweating a lot (because of hot weather or exercise) 
 Bad diarrhea 
 Vomiting (throwing up) 
 Urinating more than usual (this can happen if a person has uncontrolled diabetes or takes medicines like diuretics which cause them to urinate more often) 
 Fever

Who gets dehydration? 
Any living being can get dehydrated.  Some people are more at risk of dehydration. Elderly people, very young children, and people with chronic illnesses are at the highest risk for dehydration.

Symptoms 
Dehydrated people may have different symptoms depending on how dehydrated they are.  If dehydration is not treated, it will get worse.  As dehydration gets worse, the symptoms get more dangerous.

When people start to get dehydrated, they may have these symptoms: 
Thirst
Dry, sticky mouth
Feeling tired or sleepy
Not urinating as much as usual
Dry skin
Headache
Constipation
Feeling dizzy or lightheaded
No tears come out when the person cries

When dehydration gets severe (very bad), its symptoms include: 
 Being very thirsty
 Feeling irritable and confused (infants and children may be very fussy or get very sleepy)
 Very dry mouth and mucous membranes
 Shriveled, dry skin that is less elastic than normal (it doesn't "bounce back" when pinched into a fold)
 Urinating very little, with darkly colored urine, or not urinating at all
 Sunken eyes
 In infants, the fontanel (the soft spot on the top of the head) may be sunken
 Low blood pressure (blood pressure may also drop when the person sits up or stands up, making them feel dizzy or faint; this is called orthostasis)
 Fast heartbeat (tachycardia)
 Fast breathing (tachypnea)
 Fever
 In the most serious cases, delirium or unconsciousness

Severe dehydration is a medical emergency.  If a person with severe dehydration does not get medical treatment soon enough, they can die.

What problems may dehydration cause? 
Bad dehydration can cause serious health problems. Some of these problems are:
 Heat injury.  When it is very hot outside, or when a person is exercising a lot, fluids like water help keep the body cool.  If the person is dehydrated, their body can overheat (it gets too hot and cannot cool itself down).  This can cause heat injury.  The mildest (least serious) form of heat injury is heat cramps (cramping, or pain, in the muscles).  As heat injury gets worse, it becomes heat exhaustion.  If the body keeps overheating and the person does not get medical treatment, heat exhaustion becomes heat stroke.  Heat stroke is a very serious medical emergency.  People with heat stroke can have damage to their organs and even die if they do not get treatment quickly enough.
 Swelling of the brain (cerebral edema).  Sometimes the body tries to fix dehydration by pulling a lot of water into its cells.  If the body pulls too much water into the cells, those cells can swell and break.  If this happens to the cells in the brain, the brain can swell.  
 Seizures.  When a person gets dehydrated, their electrolytes can get out of balance.  Electrolytes are important salts in the body which carry electricity.  When electrolytes like sodium and potassium get out of balance, the body's electrical signals can get mixed up.  This can cause seizures.  
 Shock.  Fluid is an important part of human blood.  Water makes up most of blood plasma, which carries blood cells and other important things to the entire body.  If a person gets very dehydrated, the amount of blood in the body drops.  This causes low blood pressure.  If a person does not have enough blood and oxygen in their body, they can go into shock.  In medicine, this is called hypovolemic shock.
 Kidney failure.  Bad dehydration can hurt the kidneys.  When the kidneys are hurt or not working properly, they cannot remove poisons and extra fluid from the blood.

How is dehydration treated? 
To fix dehydration, a person has to replace the fluids and electrolytes that they have lost.

When dehydration is not too bad, people can usually treat themselves by drinking.   This is called oral rehydration therapy.  Stores sell special drinks like Pedialyte and Gatorade, which have both water and electrolytes in them.  People can also make their own oral rehydration solution (a drink which treats dehydration) by mixing water with sugar and salt.

People with very bad dehydration need emergency medical treatment.  They may need to get fluids intravenously (through a needle placed into a vein).  This replaces lost fluids and electrolytes much more quickly than drinking.  It is also helpful for people who are unable to drink because of nausea and vomiting from dehydration.

References 

Health problems