PERIOD definition: 1. a length of time: 2. a fixed time during the life of a person or in history: 3. in school, a…. Learn more. 13 meanings: 1. a portion of time of indefinable length 2. a. a portion of time specified in some way b. (as modifier) 3. a. Click for more definitions. PERIOD meaning: 1. a length of time: 2. a fixed time during the life of a person or in history: 3. in school, a…. Learn more. A period is all about defining when something begins and ends. Class periods usually last about forty-five minutes. Following the last food fight, there was a period of relative peace in the school. PERIOD definition: a rather large interval of time that is meaningful in the life of a person, in history, etc., because of its particular characteristics. See examples of period used in a sentence. The meaning of PERIOD is the completion of a cycle, a series of events, or a single action : conclusion. How to use period in a sentence. Synonym Discussion of Period. Menstruation (also known as a period, among other colloquial terms) is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. [1] The menstrual cycle is characterized by the rise and fall of hormones. [1].