It is constipation that does not have a physical (anatomical) or physiological (hormonal or other body chemistry) cause.
This term also describes bowel movements that are hard and/or difficult or painful to pass, even if the person has bowel movements several times per week. Constipation is very common and affects about 30% of children. Soiling describes the leakage of liquid or formed stool in underwear. Over one-half of children with functional constipation and soiling recover after 6 to 24 months of closely following treatment recommendations. The treatment includes:  Medical evaluation.  Teaching for you and your child about the constipation problem.  'Clean out' if there is an impaction of stool in the rectum. A functional gastrointestinal disorder in which constipation predominates is called chronic functional constipation. ROME II criteria for functional constipation include at least 12 weeks, which need not be consecutive, in the preceding 12 months of two or more of: 1. Straining with over one-fourth of defecations. 4. Sensation of obstruction/blockage of the anus or rectum with over one-fourth of defecations. 5. Use of manual maneuvers to facilitate over one-fourth of defecations (e.g., using fingers around or in the rectum or vagina to help get the stool out of the rectum). 6. Functional constipation as a subtype of Constipation. Constipation (medical condition): Difficult or dry bowel movements. Constipation (medical condition): Constipation is any difficulty with bowel movements or unusually dry stool. It is common in children and older age, but can affect anyone.  Chronic constipation is a common condition that is characterized by difficult, infrequent, or perceived incomplete evacuation of bowel movements. Symptoms of constipation include having less than 3 bowel movements per week, straining, hard stools, incomplete evacuation and inability to pass stool. Chronic constipation that is not due to “secondary” causes, such as other medical conditions or medications, is referred to as “functional constipation.” The three main causes of functional constipation are normal transit constipation, slow transit constipation, and defecatory or evacuation disorders. Functional Constipation. Pathogenesis and Management. Constipation is a very common clinical problem, and a questionnaire-based study in Olmsted County, Minnesota, demonstrated that up to 20% of patients report symptoms consistent with functional constipation. Approximately 40% of such patients have historic evidence of needing to strain excessively to evacuate the bowel. Second, it is important to identify evacuation disorders because a biofeedback treatment program with muscle relaxation of anal sphincters and the puborectalis can result in a 70% or greater cure rate for the Second, it is possible to have chronic functional constipation and three or more bowel movements per week. Irritable Bowel Syndrome & the MindBodySpirit Connection: 7 Steps for Living a Healthy Life with a Functional Bowel Disorder, Crohn's Disease, or Colitis (Mind-Body-Spirit Connection Series.). 4. Sensation of obstruction/blockage of the anus or rectum with over one-fourth of defecations. 5. Use of manual maneuvers to facilitate over one-fourth of defecations (e.g., using fingers around or in the rectum or vagina to help get the stool out of the rectum). 6. Functional constipation, known as chronic idiopathic constipation (CIC), is constipation that does not have a physical (anatomical) or physiological (hormonal or other body chemistry) cause. Chronic idiopathic Constipation is similar to constipation-predominant irritable bowel syndrome (IBS-C), however people with CIC do not have other symptoms of IBS, such as abdominal pain. Diagnosing CIC can be difficult as other syndromes must be ruled out as there is no physiological cause for CIC. The Rome III criteria for a diagnosis of functional constipation state that patients must have experienced two or more of the following symptoms for the past three months, and that symptoms must have begun at least six months before diagnosis: 1  straining during at least one out of four bowel movements. 4. Sensation of obstruction/blockage of the anus or rectum with over one-fourth of defecations. 5. Use of manual maneuvers to facilitate over one-fourth of defecations (e.g., using fingers around or in the rectum or vagina to help get the stool out of the rectum). 6. Constipation is a major medical problem in the United States, affecting 2% to 28% of the population. Individual patients may have different conceptions of what constipation is, and the findings overlap with those in other functional gastrointestinal disorders. 
http://www.med.umich.edu/1info/FHP/practiceguides/newconstipation/ConstipationChild.pdf https://www.sharecare.com/health/constipation/what-is-chronic-functional-constipation http://www.rightdiagnosis.com/medical/functional_constipation.htm http://uclacns.org/patients/disease-information/chronic-constipation/ http://www.medscape.com/viewarticle/717346_5 https://www.sharecare.com/health/constipation/what-is-chronic-functional-constipation https://en.wikipedia.org/wiki/Functional_constipation https://www.sharecare.com/health/constipation/what-is-chronic-functional-constipation http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2780146/
what is functional constipation