Kraepelin was the first who recognized the phenomenon of cycling in bipolar disorder in 1913. The term rapid cycling was proposed by Dunner and Fieve in 1974. Later on, Kramlinger and Post defined ultra-rapid cycling and ultradian cycling in 1996. Frequent rapid mood switches are an inherent characteristic of bipolar disorder, and rapid cycling is included as a course specifier for bipolar disorder in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). Although the terms cycle and episode are commonly used to delineate discrete periods of mood disorders, literature still suffers from the inconsistent and indistinct definition for them. Therefore, the reported prevalence of rapid cycling bipolar disorder shows a wide variability across studies, which ranges from 15% to 53.6% in adults, and 41.8% to 87.1% in children. Moreover, studies have used different definitions for rapid, ultra-rapid, and ultra-ultra-rapid/ultradian cycling, which has reduced the comparability of data across studies. However, the higher prevalence of rapid, ultra-rapid, and ultra-ultra-rapid/ultradian cycling reported in children with bipolar disorder, compared to adults, may make the pattern of cycling a key criterion in the diagnosis of early-onset bipolar disorder. Obviously, there is a significant need to stabilize a distinct specific definition for each of the terms episode, cycle, rapid cycling, ultra-rapid cycling, and ultra-ultra-rapid/ultradian cycling, based on the consensus of experts in bipolar disorder, in children and adults. In addition to increasing the value of research, using common terminologies and definitions enables comparison between studies and improves diagnostic criteria of rapid cycling bipolar disorder and its therapeutic and management plans in both the early- and adult-onset types.