1911 Encyclopædia Britannica/Interval
INTERVAL, a space left between the component parts of a continuous series, a pause in continuous action, a period of time intervening between two other points of time or chronological sequence of events. The Lat. intervallum, from which the English word has come through the French, originally meant a space between the palisades on a rampart (vallum), or between the rampart and the tents of the legionaries. In medical language “interval” is used of the intervening periods between attacks or paroxysms of a disease, particularly of the periods of a rational or normal condition of mind sometimes experienced by an insane person, a “lucid interval”; this phrase frequently occurs in legal documents from the 13th to the 15th centuries, non compos mentis sed gaudet lucidis intervallis. In music “interval” expresses the distance in pitch between two or more musical sounds (see Music). Interval, or more commonly “intervale,” is used, particularly in North America, as a geographical term for a low-lying tract of land along the banks of rivers, frequently overflowed by freshets, or more loosely for any low level land shut in by hills. This particular application, as also the form “intervale,” is due to a confusion of the termination of the word with “vale,” valley.