Why are US citizens called Americans, and since when?

The term “American” was a way to differentiate between the British who had remained in the Old World and those who had moved to the Americas.

After the colonies achieved independence from Great Britain and the fledgling United States of America was formed, the term ‘British American’ was obviously no longer needed.

The 'discovery' of America

A 1507 world map, created by cartographer Martin Waldseemuller, labelled the land visited by European explorers as America, after Italian explorer Amerigo Vespucci. Originally, the term American meant natives in these lands, but as time went on and the colonial territory began to differentiate further from their mother countries, these people too became 'British' Americans.

In Thomas Gage's The English-American: A New Survey of the West Indies, published in 1648, the first written description of European Americans was established.

Throwing off of European shackles

The United States Declaration of Independence of 1776 refers to "the thirteen united [sic] States of America", marking the first formal use of the country name. Claims on lands held by European powers were named "American possessions of Britain and Spain." There was also no reason to call oneself British anymore; the colonial power had been removed.

And so the name stuck.

Other languages

While American is the dominant demonym in English, world languages have other ways of describing them. In French, the term états-unis can be used, as well as estadounidense in Spanish, literally meaning United Statesian. Alongside these terms, forms of American is also used, Américain and americano respectively.

In British English slang the term Yankee is also used.