by QuintanaRoo (Flickr)
A dehydrated cat might have a loss of skin elasticity, dryness of the mouth, thick saliva and, in advanced cases, sunken eyeballs and shock.
A cat becomes dehydrated when the input of fluids is less than the loss.
It may be due to an insufficient water intake if ill. Fever makes things worse. Vomiting and diarrhea involve loss of fluids.
Dehydration can seriously weaken a cat. Dehydration "stops the motion of the cilia"1. These are part of the mucous membrane of the nasal cavity and a defense against infection. Dehydration reduces the effectiveness of the mucous membrane of the nasal cavity. This predisposes, for instance, the cat to bronchial infection.
A dehydrated cat can be due to acute infectious enteritis. This is an infection of the gastrointestinal tract. It causes vomiting and diarrhea coupled with a fever, rapid pulse and depression. The parvovirus (see feline viruses) that causes panleukopenia is a common cause of infectious enteritis.
Rapid dehydration can cause acute kidney failure.
Treatment is the obvious, it seems to me. It involves replacing fluids and preventing fluid loss.
Drinking water is the obvious first step. If the cat won't drink water an electrolyte solution can be given (by syringe to the cheek pouch of the mouth).
Electrolyte solutions for children are said to suitable for cats1.
Prompt veterinary attention is demanded.
Header pic: http://www.flickr.com/photos/quintanaroo/ - "..found her in the street, stuffed her in my hoodie and ran her home...to give her lots of food and water for her poor dehydrated and starved body..." (Flickr). She survived a year as far as I can tell.