One of the so called Seven Deadly Sins, lust is sexual desire in most unqualified contexts. With qualifiers and descriptors, it also includes a range of uses and idioms like related to forms of ambition. In abstract it's really a personal desire, a psychic obsession in most forms. In old moral tales, lust is always a negative attribute, a crime by definition. It's usually a standby in modern media, too, the basis of relationships and the thematic glue for soap operas. In 'polite' Western literature during the Christian era, lust was often implied in older forms of fiction rather than stated up to the 19th century.