Mule

Definition
noun, plural: mules
(zoology) A sterile hybrid offspring, especially of a male donkey and a female horse
(botany) Any sterile hybrid plant
Supplement
In biology, a mule generally pertains to a hybrid offspring that is sterile. For instance, a cross between a male donkey (jack) and a female horse (mare) results in a progeny (male or female) that is sterile. Although donkeys and horses belong to the same genus, Equus, they are two different species. Horses are taxonomically designated as Equus ferus caballus whereas donkeys are designated as E. africanus. The two species have different number of chromosomes. A donkey has 62 chromosomes whereas a horse has 64 chromosomes. Mules are much used as draught animals. They are more hardy and long-lived than horses, and described to be more intelligent than donkeys. A reciprocal cross, i.e. a cross between a male horse (stallion) and a female donkey (jenny), results in a similarly sterile offspring, called a hinny.
Other examples of animal mules are the offspring from a cross between the canary and some other finch, and that between a meat ram and a hardy mountain ewe. In plants, a mule also pertains to a hybrid plant, especially one that is sterile. A plant or vegetable produced by impregnating the pistil of one species with the pollen or fecundating dust of another.
See also:

• Cross
• Hybrid