Hardcore can be used to describe a number of genres including:
1) The genre of hardcore punk as well as an umbrella term encompassing various offshoots and derivatives (metalcore, crossover thrash, powerviolence etc). It can also be used as an adjective rather than a genre, often to describe a metalcore band that sounds heavier or more "hardcore" than other bands.
2) A genre of electronic music, with has some characteristics like being harder than the other electronic music genres and the songs having higher bpms, sometimes called hardcore techno or gabber.
3) The genre of Hardcore Rap or Hardcore Hip Hop. .