any of the largest and most powerful landholding magnates in Japan from about the 10th century until the latter half of the 19th century. The Japanese word daimyo is compounded from dai (“large”) and myō (for myōden, or “name-land,” meaning “private land”).

Upon the breakdown of the system of public-land domain in Japan after the 8th century, private landholdings of various sorts came into being. These holdings were first consolidated into estates (shōen) organized under the authority of the civil nobility and religious establishments, and they remained within the framework of imperial government. As the military class (samurai) (buke, or samurai) increased in numbers and importance during the 11th and 12th centuries, the term daimyo came to be applied to those military lords who began exercising territorial control (and later proprietary rights) over the various private estates into which the country had become divided.

In the 14th and 15th centuries the so-called shugo daimyo arose. These daimyo were appointed as military governors ( shugo) under the Ashikaga shoguns (hereditary military dictators), and they held legal jurisdiction over areas as large as provinces. The shugo daimyo's private landholdings were quite limited, however, and these daimyo gained much of their income from levying taxes on the cultivated lands owned by civil aristocrats and religious establishments. In the second half of the 15th century the shugo daimyo were supplanted by the Sengoku daimyo (i.e., daimyo of the Sengoku, or “Warring States” period); these military lords held small but consolidated domains in which all the land belonged to themselves or was held in fief by their vassals. By the late 15th century the Sengoku daimyo had divided Japan into a series of small, belligerent states as each individual daimyo competed for the control of more territory. The Sengoku daimyo built castles in the hill country from which they controlled their vassals, who likewise were petty landowners with castles.

In the 16th century the Sengoku daimyo fought among themselves constantly, and a process of consolidation ensued, with fewer and fewer daimyo emerging from the local wars and each holding more and more territory. In 1568 Oda Nobunaga began the movement of decisive military conquest over the daimyo that was later carried on by Toyotomi Hideyoshi and completed in 1603 by Tokugawa Ieyasu. By this time roughly 200 daimyo had been brought under the hegemony of the Tokugawa family, the head of which served as shogun. In the 16th century the term daimyo became limited in its application to territorial lords having lands (han) assessed at 10,000 koku (1 koku = 5 bushels) or more of annual grain production.

The daimyo of the Tokugawa, or Edo, period (1603–1867) served as local rulers in the three quarters of the country not held as grain-producing (granary) land by the shogunate, or bakufu (literally, “tent government”). Daimyo were joined to the shogun by oath and received their lands as grants under his vermilion seal in a governing system called the bakuhan. Daimyo were classed according to their relationships to the shogun as kinsmen (shimpan), hereditary vassals (fudai), and less-trusted allies (tozama; meaning “outsiders”).

The kinsei (“early modern”) daimyo, as the daimyo of the Tokugawa period were called, differed from their predecessors in being more nearly petty monarchs within their domains. Their own samurai vassals, or retainers, were no longer holders of outlying castles but had been drawn off the land and brought into garrison residence at the daimyo's own great castle, which alone stood at the centre of the domain. The daimyo divided his domain between his own personal granary land and the land on which his chief retainers were enfeoffed. Normally his granary land amounted to from 30 to 40 percent of the whole. The daimyo's retainers were divided between fief holders and salaried retainers. All daimyo worked to convert their enfeoffed vassals to the enforced dependence of the salaried status, and by the 18th century most fiefs had been absorbed under the daimyo's expanding authority.

The daimyo used his band of retainers (kashindan) to administer his domain. A council of elders (karō) held the responsibility for policy and the superintendence of other officials, among whom were the heads of military units, superintendents of the castle town, rural administration, finance, security, public works, religious affairs, education, a secretariat, and many other specific posts. Within their domains the greater daimyo had considerable freedom, even to the point of issuing their own paper currency with the shogun's permission.

Daimyo came under the centralizing influence of the Tokugawa shogunate in two chief ways. In a sophisticated form of hostage-taking that was used by the shogunate, the daimyo were required to alternate their residence between their domains and the shogun's court at Edo (now Tokyo) in a system called sankin kōtai. Second, since shogunate law took precedence within the country, the daimyo adopted within their domains the general principles of Tokugawa law and bureaucratic procedure.

By the end of the Tokugawa regime, the daimyo had become removed from the actualities of government and basically served as aristocratic figureheads in their domains. This in part accounted for the success of the effort to abolish the daimyo. In 1868 the shogunate was abolished, and in 1869 the daimyo were obliged to turn back their land patents to the emperor, being made instead governors of territories corresponding roughly to their former domains. In 1871 the domains were abolished, and the former daimyo were converted into a pensioned nobility residing in Tokyo.