From Wikipedia: Frederick, Hereditary Prince of Denmark (1753-1805) was heir presumptive to the thrones of Denmark and Norway. He was the only son of King Frederick V.
Please explain the title Hereditary Prince of Denmark.
This title is usually given to those, who are first in line of succession, but are not the son/daughter of the current King and thus will be bypassed by a son/daughter of the current King and/or are the daughter of the current King, but can be bypassed by a younger brother.
Frederick was the son of Frederick V. by his second marriage to Juliana of Brunswick. Thus, when his elder half brother became King, he became the Hereditary Prince. And Wikipedia is wrong, he was not the only son of Frederick V.