Sugar beet is a crop with a large round root. It is grown for the sugar which can be obtained from this root.
sugar beet in British English
noun
a variety of the plant Beta vulgaris that is cultivated for its white roots from which sugar is obtained
Compare sugar cane
sugar beet in American English
a variety or cultivar of the common beet (Beta vulgaris) having a root with white flesh and a high sugar content, grown commercially as a source of sugar