Jailbait is American English slang for a person who is younger than the legal age of consent for sexual activity, but physically mature enough to be mistaken for an adult and be considered sexually desirable.
Synonyms for the term include: nymph, nymphet or nymphette. The French mainly use the term lolita.
The term literally denotes a person younger than the age of consent who could be used to trap a sexual criminal.
Although, it should be noted that the term is highly nuanced due to its criminal law connotations; therefore, the specific sort of person to which the term can refer to varies by jurisdiction and often even by the person who uses the term.