In academic publishing, an embargo is a period during which access is not allowed to certain types of users. The purpose of this is to protect the revenue of the publisher. Various types exist: * A moving wall is a fixed period of months or years. (See below.) * A fixed date is a particular time point that does not change. * A current year (or other period) is setting a time point on Jan. 1 of the current year, so that all material earlier than that is available. Though fixed during the year, it will change each year.