Time between WWI and WWII

Longman Dictionary of Contemporary English Word definitions in Longman Dictionary of Contemporary English
adjective EXAMPLES FROM CORPUS ▪ Plenty of reckless people found themselves enthusing about dictators during the interwar years. ▪ The medieval, Renaissance and modern collections cover more than a millennium, from the Dark Ages to the interwar period.

Wiktionary Word definitions in Wiktionary
a. Relating to the period of time between two wars, usually the two world wars (1919 - 1939). alt. Relating to the period of time between two wars, usually the two world wars (1919 - 1939).