A crown prince or hereditary prince is the heir apparent to the throne in a royal or imperial monarchy. The female form of the title is crown princess, which may refer either to an heiress apparent or, especially in earlier times, to the wife of the person styled crown prince.
Crown prince as a descriptive term has been used throughout history for the prince who is first-in-line to a throne and is expected to succeed (i.e. the heir apparent), barring any unforeseen future event preventing this. In certain monarchies, a more specific substantive title may be accorded and become associated with the position of heir apparent (e.g. Prince of Wales in the United Kingdom). In these monarchies, the term crown prince may be used less often than the substantive title (or never).
Until the late twentieth century, no modern monarchy adopted a system whereby females would be guaranteed to succeed to the throne (i.e. absolute primogeniture). A crown princess would therefore be more likely to refer to the spouse of a crown prince. She would be styled crown princess, not in her own right but by courtesy.