A hundred is an administrative division that is geographically part of a larger region. It was formerly used in England, Wales, some parts of the United States, Denmark, Sweden, Finland, Norway, and in Cumberland County in the British Colony of New South Wales. It is still used in other places, including in Australia (in South Australia and the Northern Territory). Other terms for the hundred in English and other languages include wapentake, herred (Danish and Bokmål Norwegian), herad (Nynorsk Norwegian), härad or hundare (Swedish), Harde (German), hiird (North Frisian), kihlakunta (Finnish), and cantref (Welsh). In Ireland, a similar subdivision of counties is referred to as a barony, and a hundred is a subdivision of a particularly large townland (most townlands are not divided into hundreds).

The origin of the division of counties into hundreds is described by the Oxford English Dictionary as "exceedingly obscure". It may once have referred to an area of 100 hides; in early Anglo-Saxon England a hide was the amount of land farmed by and required to support a peasant family, but by the 11th century in many areas it supported four families. Alternatively the hundred may have been an area originally settled by one "hundred" men at arms, or the area liable to provide one "hundred" men under arms. In this early medieval use, the number term "hundred" can be unclear, meaning the "short" hundred (100) or in some contexts the long hundred of 120.

There was an equivalent traditional Germanic system. In Old High German a huntari is a division of a gau, but the Oxford English Dictionary states that the link between the two is not established.

From the 11th century in England—and to a lesser extent from the 16th century in Wales—to the middle of the 19th century, the annual assemblies had varying degrees of power at a local level in the feudal system. Of chief importance was their more regular use for taxation, and six centuries of taxation returns for the divisions survive to this day.

Groupings of divisions, small shires, were used to define parliamentary constituencies from 1832 to 1885. On the redistribution of seats in 1885 a different county subdivision, the petty sessional division, was used. Hundreds were also used to administer the first five national censuses from 1801 to 1841.

The system of county divisions was not as stable as the system of counties being established at the time, and lists frequently differ on how many hundreds a county had. In many parts of the country, the Domesday Book contained a radically different set of divisions from that which later became established. The numbers of divisions in each county varied widely. For example Leicestershire had six (up from four at Domesday), whereas Devon, nearly three times the size, had 32.

By the end of the 19th century, several single-purpose subdivisions of counties, such as poor law unions, sanitary districts, and highway districts, had sprung up, which, together with the introduction of urban districts and rural districts in 1894, mostly replaced the role of the parishes, and to a lesser extent the less extensive role of hundreds. The division names gave their name to multiple modern local government districts.

In southern and western England, a hundred was the division of a shire for military and judicial purposes under the common law, which could have varying extent of common feudal ownership, from complete suzerainty to minor royal or ecclesiastical prerogatives and rights of ownership. Until the introduction of districts by the Local Government Act 1894, hundreds were the only widely used assessment unit intermediate in size between the parish, with its various administrative functions, and the county, with its formal, ceremonial functions.