Constipation is not a disease by itself, but a symptom of some other underlying problem. Animals are very individual when it comes to the number of bowel movements per day. There is no set number of bowel movements per day representing a normal function of the intestine. A normal animal usually passes stool once or twice  a day in average.
Constipation is a condition in which there is a reduction in the frequency of bowel movements and the consistency of the stool is dry and hard.
Constipation can appear in any type of animal but it is much more common in cats than in dogs.
The most common reasons for constipation are: dehydration, changes in the diet (the food has to be rich enough in fiber to produce normal stool), painful defecation, such as anal gland infection that leads to reluctance to defecate. Cats are very finicky about their litter box.If the litter box is not clean enough, changes in the type of litter, or changes in the litter box location, can make the cat refuse to use the litter box and later to constipation. Other reasons for constipation include certain drugs side effects, anatomical changes in the intestine that cause partial obstruction such as polyps or tumors,metabolic or hormonal imbalances (for example in Hypothyroidsm) pelvic injuries, prostate gland enlargement and more. Very frequently the exact reason for the constipation is unknown and there is no apparent cause, these cases are called in medical terminology- idiopathic.
The hallmark of constipation is straining to defecate. When the cat does pass stool, it may be watery at first and then followed by dry feces. The defecation may be accompanied by crying in pain. Defecation outside of the litter box can also be a sign that something is wrong. In advanced cases of the condition, other symptoms such as vomiting, lethargy, hunched back, appetite loss and weight loss can appear.
The diagnosis of constipation is based on a physical exam, including palpation of the abdomen. X rays confirm the presence of constipation and are also done in attempt to find out the cause. Other tests such as a blood test, abdominal ultrasound or other tests may be required in the diagnosis process of isolating the underlying cause for the constipation.
The treatment of constipation consists of laxatives and a high fiber diet. Severe cases may need to be treated by an enema, a penetrative procedure that is done under sedation/anesthesia in which the feces is evacuated from the colon.
In order to try to prevent constipation you should feed your pet a diet that is high in fiber, make sure your pet has constant access to fresh water at all times. Cat owners should keep the litter box clean. If your cat has long hair, brush it often. If your cat tends to swallow hair and form hairballs, you can treat it with a mild lubricant paste that can help it safely pass the hairball.
Chronic constipation may lead to permanent dilatation of the large intestine, a condition called Megacolon. This condition is found primarily in cats but can also be found in dogs. The treatment for megacolon is more complicated and in addition to the constipation treatment, other drugs that affect the intestinal innervation and motility are also prescribed.
If you recognize any of the constipation symptoms described, take your pet to be seen by your vet and get help for it.