mouth, opening, entrance, door, There are literally hundreds of words derived from this character. Here are a few from my pop-up dictionary. 入口【rùkǒu】 enter the mouth; entrance. 出口【chūkǒu】 speak; utter; exit; export. 口【kǒu】 mouth; opening; entrance; mouth; a gateway of the Great Wall (often used in place names); cut; hole; the edge of a knife; the edge of a draft animal; <measure> . 口碑【kǒubēi】 public praise. 口才【kǒucái】 eloquence. 口臭【kǒuchòu】 halitosis; bad breath. 口袋【kǒudài】 pocket, sack. 口红【kǒuhóng】 lipstick. 口信【kǒuxìn】 oral message. 住口【zhùkǒu】 shut up; stop talking. 口语【kǒuyǔ】 spoken language.

it can be a part of other words and the counter for family meters example 我 家 有 六 口 人。

口头警告

Thanks for the answers ^_^

beside uncle said you can also use 人口 population 戶口 passport 口號 slogan 口試 oral test 可口 tasty dilicius 口口香 very good taste ............