The Oxford English Dictionary (OED) is the principal historical dictionary of the English language, published by Oxford University Press (OUP). It traces the historical development of the English language, providing a comprehensive resource to scholars and academic researchers, as well as describing usage in its many variations throughout the world.

Work began on the dictionary in 1857, but it was only in 1884 that it began to be published in unbound fascicles as work continued on the project, under the name of A New English Dictionary on Historical Principles; Founded Mainly on the Materials Collected by The Philological Society. In 1895, the title The Oxford English Dictionary was first used unofficially on the covers of the series, and in 1928 the full dictionary was republished in 10 bound volumes. In 1933, the title The Oxford English Dictionary fully replaced the former name in all occurrences in its reprinting as 12 volumes with a one-volume supplement. More supplements came over the years until 1989, when the second edition was published, comprising 21,728 pages in 20 volumes. Since 2000, compilation of a third edition of the dictionary has been underway, approximately half of which was complete by 2018.

The first electronic version of the dictionary was made available in 1988. The online version has been available since 2000, and by April 2014 was receiving over two million visits per month. The third edition of the dictionary is expected to be available exclusively in electronic form; the Chief Executive of Oxford University Press has stated that it is unlikely that it will ever be printed.