The Great Satan is a derogatory epithet

Epithet

An epithet or byname is a descriptive term accompanying or occurring in place of a name and having entered common usage. It has various shades of meaning when applied to seemingly real or fictitious people, divinities, objects, and binomial nomenclature. It is also a descriptive title...

 for the United States of America in some Iranian

Politics of Iran

The politics of Iran take place in a framework of theocracy guided by an Islamist ideology. The December 1979 constitution, and its 1989 amendment, define the political, economic, and social order of the Islamic Republic of Iran, declaring that Shi'a Islam of the Twelver school of thought is...

 foreign policy

Foreign policy

A country's foreign policy, also called the foreign relations policy, consists of self-interest strategies chosen by the state to safeguard its national interests and to achieve its goals within international relations milieu. The approaches are strategically employed to interact with other countries...

 statements. Occasionally, these words have also been used toward the government of the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

.

The term was originally used by Iranian leader Ruhollah Khomeini

Ruhollah Khomeini

Grand Ayatollah Sayyed Ruhollah Musavi Khomeini was an Iranian religious leader and politician, and leader of the 1979 Iranian Revolution which saw the overthrow of Mohammad Reza Pahlavi, the Shah of Iran...

 in his speech on November 5, 1979 to describe the United States whom he accused of imperialism

Imperialism

Imperialism, as defined by Dictionary of Human Geography, is "the creation and/or maintenance of an unequal economic, cultural, and territorial relationships, usually between states and often in the form of an empire, based on domination and subordination." The imperialism of the last 500 years,...

 and the sponsoring of corruption throughout the world.

Ayatollah Khomeini also occasionally used the terms Iblis (Diabolis - the primary devil in Islam) to refer to the United States and other Western countries.
Iranians have considered the United States and the United Kingdom as Imperialist states, who have a long history of interfering in Iran's internal affairs.