The noun 美国人 (Měiguórén) means "American" (a person from the United States). It follows the standard pattern of country name + 人 to form nationality words: 美国 (America) + 人 (person). This pattern works for most nationalities in Chinese.
我的邻居是美国人,他中文说得很好。
Wǒ de línjū shì Měiguórén, tā Zhōngwén shuō de hěn hǎo.
My neighbor is American, and he speaks Chinese very well.
这家餐厅的老板是美国人,但他做的中餐特别地道。
Zhè jiā cāntīng de lǎobǎn shì Měiguórén, dàn tā zuò de zhōngcān tèbié dìdao.
The owner of this restaurant is American, but the Chinese food he makes is especially authentic.
很多美国人喜欢在周末看橄榄球比赛。
Hěn duō Měiguórén xǐhuān zài zhōumò kàn gǎnlǎnqiú bǐsài.
Many Americans like to watch football games on weekends.