Whose skull does hamlet speak to in the graveyard?

In the graveyard scene of Shakespeare's "Hamlet," the title character speaks to the skull of Yorick, the former court jester. Hamlet reminisces about Yorick's playful nature and their past interactions, reflecting on themes of mortality and the inevitability of death. This moment serves as a poignant turning point for Hamlet, deepening his contemplation of life, death, and the futility of existence.