Ideologically, the Victorian era witnessed resistance to the rationalism that defined the Georgian period and an increasing turn towards romanticism and mysticism with regard to religion, social values, and arts. In international relations, the supremacy of the Royal Navy helped maintain a period of relative peace among the great powers (Pax Britannica) as well as economic, colonial, and industrial consolidation and expansion, a notable exception being the Crimean War (1853–56). Britain embarked on global imperial expansion, particularly in Asia and Africa, which made the British Empire the largest empire in history.
Domestically, the political agenda was increasingly liberal, with a number of shifts in the direction of gradual political reform, industrial reform, and the widening of the voting franchise. There were unprecedented demographic changes: the population of England and Wales almost doubled from 16.8 million in 1851 to 30.5 million in 1901, and Scotland's population also rose rapidly, from 2.8 million in 1851 to 4.4 million in 1901. However, Ireland's population decreased sharply, from 8.2 million in 1841 to less than 4.5 million in 1901, mostly due to emigration and the Great Famine. Between 1837 and 1901 about 15 million emigrated from Great Britain, mostly to the United States, Canada, South Africa, New Zealand, and Australia.
The two main political parties during the era remained the Whigs/Liberals and the Conservatives; by its end, the Labour Party had formed as a distinct political entity. These parties were led by such prominent statesmen as Lord Melbourne, Sir Robert Peel, Lord Derby, Lord Palmerston, Benjamin Disraeli, William Gladstone, and Lord Salisbury. The unsolved problems relating to Irish Home Rule played a great part in politics in the later Victorian era, particularly in view of Gladstone's determination to achieve a political settlement in Ireland.
In the strictest sense, the Victorian era covers the duration of Victoria's reign as Queen of the United Kingdom of Great Britain and Ireland, from her accession on 20 June 1837—after the death of her uncle, William IV—until her death on 22 January 1901, after which she was succeeded by her eldest son, Edward VII. Her reign lasted for 63 years and seven months, a longer period than any of her predecessors. The term 'Victorian' was in contemporaneous usage to describe the era. The era has also been understood more extensively as a period that possessed sensibilities and characteristics distinct from those adjacent, in which case it is sometimes dated to begin before Victoria's accession—typically from the passage of or agitation for (during the 1830s) the Reform Act 1832, which introduced a wide-ranging change to the electoral system of England and Wales. Definitions according to a distinct sensibility or politics have also created scepticisim about the worth of the label "Victorian", though there have equally been defences of it as a marker of time.
Michael Sadleir was insistent that "in truth the Victorian period is three periods, and not one". He distinguished early Victorianism - the socially and politically unsettled period from 1837-1850 - and late Victorianism (from 1880 onwards), with its new waves of Aestheticism and Imperialism, from the Victorian heyday: mid-Victorianism, 1851 to 1879. He saw the latter period as characterised by a distinctive mixture of prosperity, domestic prudery, and complacency - what G. M. Trevelyan similarly called the "mid-Victorian decades of quiet politics and roaring prosperity".