A crack or tear in the lining of the anus is known as:

An anal fissure refers specifically to a crack or tear in the lining of the anus. This condition is typically caused by trauma to the anal canal, which can occur during bowel movements, particularly if the stool is hard or large. Anal fissures can lead to significant discomfort and may cause pain during and after bowel movements, as well as bleeding.

In contrast, the other options represent different medical conditions. Constipation is a common digestive issue characterized by infrequent or difficult bowel movements. Crohn's disease is a type of inflammatory bowel disease that can cause inflammation in various parts of the gastrointestinal tract, including the anus, but does not specifically refer to a tear. Diverticulitis involves the inflammation or infection of pouches that can form in the walls of the colon and is not related to an anal tear. Therefore, the definition provided by the term "anal fissure" aligns perfectly, as it directly describes the condition in question.