What does 'American exceptionalism' mean
It's the belief that the US has a special place among the nations of the world - that it has a particular role and obligations to be a beacon for the free world. The phrase stems from constitutional writer Alexis de Toqueville, writing in 1840, who said:
"The position of the Americans is therefore quite exceptional, and it may be believed that no other democratic people will ever be placed in a similar one."
Alexis de Tocqueville, Democracy in America (1840), part 2, p. 36