Obstructed defecation syndrome (abbreviated as ODS, with many synonymous terms) is a major cause of functional constipation (primary constipation), of which it is considered a subtype. It is characterized by difficult and/or incomplete emptying of the rectum with or without an actual reduction in the number of bowel movements per week. Normal definitions of functional constipation include infrequent bowel movements and hard stools. In contrast, ODS may occur with frequent bowel movements and even with soft stools, and the colonic transit time may be normal (unlike slow transit constipation), but delayed in the rectum and sigmoid colon.

Constipation is usually divided into two groups: primary and secondary. Primary constipation is caused by disrupted regulation of neuromuscular function of in the colon and the rectum, and also disruption of brain–gut neuroenteric function. Secondary constipation is caused by many other different factors such as diet, drugs, behavioral, endocrine, metabolic, neurological, and other disorders. There are main subtypes of primary constipation which are recognized, although overlap exists (see: Co-existence of different constipation subtypes): dyssynergic defecation, slow transit constipation (colonic dysmotility) and irritable bowel syndrome with constipation.

Obstructed defecation is one of the causes of chronic constipation. ODS is a loose term, consisting of a constellation of possible symptoms, caused by multiple, complex and poorly understood disorders which may include both functional and organic disorders. The topic of defecation disorders is very complicated, and there is a lot of confusion regarding terminology and classification in published literature. Occasionally some sources inappropriately treat ODS as a synonym of anismus. Although anismus is a major cause of ODS, there are other possible causes. Other authors use the term ODS to refer to defecatory dysfunction in the absence of any pathological findings (that is, a purely functional disorder). Furthermore, many different terms have been used for ODS, which appear to refer to the same clinical entity. The term ODS does not appear in the ICD-11 and Rome-IV classifications, which both instead refer to "functional defecation disorders". One publication criticized such classifications as being ambiguous and based on symptoms rather than distinct etiopathological entities. The authors suggested that "evacuation disorders" be used as a descriptive term, which would be subclassified to include all possible factors that may be contributory to the symptoms.

In 2001, the American Society of Colon and Rectal Surgeons (ASCRS), the Colorectal Surgical Society of Australia, and the Association of Coloproctology of Great Britain and Ireland published a consensus statement which covered definitions relevant to this topic. A revised consensus statement was published by the ASCRS in 2018. Wherever possible, this article generally follows the definitions and terminology of the 2018 consensus statement, wherein ODS is defined as "a subset of functional constipation in which patients report symptoms of incomplete rectal emptying with or without an actual reduction in the number of bowel movements per week." Functional constipation is usually defined as infrequent bowel movements and hard stools. In contrast, ODS may occur with frequent bowel movements and even with soft stools, and the colonic transit time may be normal (unlike slow transit constipation).

The ODS may or may not co-exist with other functional bowel disorders, such as slow transit constipation or irritable bowel syndrome. Of all cases of primary constipation, it is reported that 58% are dyssynergic defecation, 47% are slow transit constipation and 58% are irritable bowel syndrome. Significant overlap exists. For example, approximately 60% of patients with dyssynergic defecation also have STC. In a study of 1,411 patients with chronic constipation referred to a tertiary center, 68% had normal transit constipation, 28% had evacuation disorders and less than 1% had slow transit constipation without any evacuation disorder.

The term "obstructed defecation syndrome" does not appear in ICD-11. However, the following entries are present, as well as separate codes for most of the individual organic lesions listed in this article:

The term "obstructed defecation syndrome" does not appear in the Rome IV classification. However, diagnostic criteria for functional defecation disorders are listed. According to Rome-IV, this is defined as "features of impaired evacuation" during repeated attempts to defecate. To qualify for this diagnosis, patients must meet the Rome diagnostic criteria for functional constipation or irritable bowel syndrome with constipation (IBS-C). Furthermore, 2 of the following 3 tests must show abnormal results: balloon expulsion test, anorectal manometry or anal surface electromyography, or imaging (e.g. defecography). Two subcategories exist within the functional defecation disorders category: Inadequate defecatory propulsive (F3a) and Dyssynergic defecation (F3b). These are defined as "Inadequate propulsive forces as measured with manometry with or without inappropriate contraction of the anal sphincter and/or pelvic floor muscles", and "Inappropriate contraction of the pelvic floor as measured with anal surface EMG or manometry with adequate propulsive forces during attempted defecation" respectively. The subcategories F3a and F3b are defined by age- and gender-appropriate normal values for the technique. For all of these Rome-IV diagnoses, diagnostic criteria must have been fulfilled for the last 3 months with symptom onset at least 6 months prior to diagnosis.

There is a constellation of possible symptoms.