ギリシャ語のmonas, singleから、語源的にはmonad, monasteryと同じ。つまり、一人で生活する者で、修道士、僧侶、修行僧などに使われる。
In England, before the Reformation, the term was not applied to the members of the mendicant orders, who were always called friars. From the 16th c. to the 19th c., however, it was usual to speak of the friars as a class of monks. In recent times the distinction between the terms has been carefully observed by well-informed writers. In French and Ger. the equivalent of monk is applied equally to 'monks' and 'friars.' [OED]