Class S , or S kankei, abbreviated either as S or Esu (エス), is an early twentieth-century Japanese wasei-eigo term used to refer to romantic friendships between girls. The term is also used to designate a genre of girl's fiction which tells stories about the same, typically focused on senpai and kōhai relationships wherein one girl is senior in age or position to the other. The "S" is an abbreviation that can stand for "sister", "shōjo" , "sex", "schön", and "escape".
Hotaru Kari (ほたる狩り, "Firefly Hunting"), illustration by Shigeru Sudō [ja], 1926
A romantic friendship, passionate friendship, or affectionate friendship is a very close but typically non-sexual relationship between friends, often involving a degree of physical closeness beyond that which is common in contemporary Western societies. It may include, for example, holding hands, cuddling, hugging, kissing, giving massages, or sharing a bed, without sexual intercourse or other sexual expression.
Shimer College founders Cindarella Gregory and Frances Shimer in 1869; their extremely close relationship has been characterized as a "passionate friendship".