Hardcast is Magic: The Gathering jargon used to describe the playing of a spell for its actual mana cost. This is opposed to using a different method of casting the spell or putting a permanent into play. For example, playing Force of Will by paying for it is called hardcasting, while playing it by paying 1 life and exiling a blue card is called pitching.
The capability or act of hardcasting may describe multiple things, such as the possibility of producing large quantities of mana over the course of a game, the ability to do so quickly, the lack of other means to put a card into play either because the deck doesn't have them or the player didn't draw them, or simply an exclamation of the length of a specific game.
The inability of some decks, such as Reanimator, Tooth and Nail, Hypergenesis or decks based on Sneak Attack, to hardcast a portion of its spells may be an exploitable weakness, as the creatures they want to put into play are dead cards without the respective enablers.