(A trademark for) a breakfast cereal made from grains of rice which are steamed, sweetened, and then heated rapidly in the manner of popcorn; (also in singular) a piece of this cereal.
Origin
1920s; earliest use found in The Chicago Tribune.
Definition of Rice Krispies in US English:
Rice Krispies
noun
(A trademark for) a breakfast cereal made from grains of rice which are steamed, sweetened, and then heated rapidly in the manner of popcorn; (also in singular) a piece of this cereal.