The definition of constipation includes the following:

• infrequent bowel movements (typically three times or fewer per week)
• difficulty during defecation (straining during more than 25% of bowel movements or a subjective sensation of hard stools), or
• The sensation of incomplete bowel evacuation.

The following are some of the casues of constipation:
Primary
Primary or functional constipation is ongoing symptoms for greater than six months not due to any underlying cause such as medication side effects or an underlying medical condition. It is not associated with abdominal pain thus distinguishing it from irritable bowel syndrome. It is the most common cause of constipation.

Diet
Constipation can be caused or exacerbated by a low fiber diet, low liquid intake, or dieting.

Medication
Many medications have constipation as a side effect. Some include: opioids (e.g. common codeine pain killers), diuretics, antidepressants, antihistamines, antispasmodics, anticonvulsants, and aluminum antacids

Metabolic & muscular
Metabolic and endocrine problems which may lead to constipation include: hypercalcemia, hypothyroidism, diabetes mellitus, cystic fibrosis, and celiac disease. Constipation is also common in individuals with muscular and myotonic dystrophy.

Structural and functional abnormalities
Constipation has a number of structural (mechanical, morphological, anatomical) causes, including: spinal cord lesions, Parkinsons, colon cancer, anal fissures, proctitis, and pelvic floor dysfunction. Constipation also has functional (neurological) causes, including anismus, descending perineum syndrome, and Hirschsprung's disease. In infants, Hirschsprung's disease is the most common medical disorder associated with constipation. Anismus occurs in a small minority of persons with chronic constipation or obstructed defecation.

Psychological
Voluntary withholding of the stool is a common cause of constipation. The choice to withhold can be due to factors such as fear of pain, fear of public restrooms, or laziness.