The United States is not a Democracy it's a Republic.
A republic is a form of government in which the head of state is not a monarch and the people (or at least a part of its people) have an impact on its government. The word 'republic' is derived from the Latin phrase res publica which can be translated as "public affairs".
Both modern and ancient republics vary widely in their ideology and composition. The most common definition of a republic is a state without a monarch. In republics such as the US and France the executive is legitimated both by a constitution and by popular suffrage. In the United States Founding Fathers like James Madison defined republic in terms of representative democracy as opposed to only having direct democracy, and this usage is still employed by many viewing themselves as "republicans". In modern political science, republicanism refers to a specific ideology that is based on civic virtue and is considered distinct from ideologies such as liberalism.
Most often a republic is a sovereign country, but there are also subnational entities that are referred to as republics. For instance, Article IV of the Constitution of the United States "guarantee[s]to every State in this Union a Republican form of Government." The Soviet Union was a single nation composed of distinct and legally sovereign Soviet Socialist Republics.