As a Noun: 1. Each and every week which is the fourth week in a cycle (usually of four weeks), or each and every unit of time which is the fourth unit in a cycle. As Modifier: 1. Used in a sentence (immediately next to a verb, or at the beginning or end of the sentence) to mean that the action happens every fourth unit of time which has already been established (day, week, month, semester, year, etc.). Most commonly refers to weeks, i.e., every four weeks or monthly.