Yrast ( , ) is a technical term in nuclear physics that refers to a state of a nucleus with a minimum of energy (when it is least excited) for a given angular momentum. "Yr" is a Swedish adjective sharing the same root as the English "whirl". "Yrast" is the superlative of "yr" and can be translated "whirlingest", although it literally means "dizziest" or "most bewildered". The yrast levels are vital to understanding reactions, such as off-center heavy ion collisions, that result in high-spin states.