Tricyclic refers to a three-ring structure or system, often used in chemistry and pharmacology. In this context, it describes a type of organic compound that contains three condensed rings (usually with five or six members each) within its molecular structure. These types of compounds are commonly found in natural products such as antibiotics and anti-inflammatory drugs like the tricyclic antidepressants, which include imipramine, amitriptyline, and nortriptyline.