Mule, a French word, is a style of shoe that is backless and often closed-toed. Mules can be any heel height - from flat to high. The style is predominantly (but not exclusively) worn by women.^{[citation needed]}

The term derives from the Ancient Roman mulleus calceus a red or purple shoe worn by the three highest magistrates, although there is little indication of any structural resemblance.

High-heeled mules were a popular indoor shoe style of the 18th century, influenced by the patten, a backless overshoe of the 16th century. By the early twentieth century, mules were often associated with prostitutes.

In the early 1950s, Marilyn Monroe popularized the shoe and helped to break its poor reputation.

Mules experienced some popularity in the 1950s and early 1960s, and were seen in 1970s almost exclusively in the form of open-back Scandinavian clogs, but then re-emerged in the early 1990s, especially in its open-toed form (the "slide"), and began to dominate^{[citation needed]} the shoe market for women.

A mule is the offspring of a male donkey and a female horse. Horses and donkeys are different species, with different numbers of chromosomes. Of the two F1 hybrids between these two species, a mule is easier to obtain than a hinny (the offspring of a male horse and a female donkey). While there is no known instance of a male mule siring offspring, female mules have on very rare occasion given birth to viable offspring. The size of a mule and work to which it is put depends largely on the breeding of the mule's dam. Mules can be lightweight, medium weight, or even, when produced from draught horse mares, of moderately heavy weight.

An aficionado of the mule claims that they are "more patient, sure-footed, hardy and long-lived than horses, and they are considered less obstinate, faster, and more intelligent than donkeys."

A female mule that has estrus cycles and thus, in theory, could carry a fetus, is called a "molly" or "Molly mule," though the term is sometimes used to refer to female mules in general. Pregnancy is rare, but can occasionally occur naturally as well as through embryo transfer. One of several terms for a gelded mule is a "John mule."