美国 is used here to mean the United States, but in other lessons it is used to refer to the American continent, including Canada and South and Central America. Is there a precise definition, or does it depend on context?
Fun tips: America was transliterated 亜米利加 first in Japanese. Chinese did alike but they spelled 亜美利加 - 美 means beauty (originally "as splendid as large大 sheep羊 for sacrifice). Literally "美国" means "beautiful country".