美洲 refers to both continents collectively. Use 北美洲 (Běiměizhōu) for North America and 南美洲 (Nánměizhōu) for South America. 美国 (Měiguó) specifically means the USA.
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Each continent ends in 洲: 亚洲 (Asia), 欧洲 (Europe), 非洲 (Africa), 大洋洲 (Oceania), 南极洲 (Antarctica). The first character is a shortening of the full name.
