What is a aspeech delivered by a character alone onstage revealing his or her very private thoughts motives or state of mind?

A speech delivered by a character alone onstage revealing their private thoughts, motives, or state of mind is called a "soliloquy." This dramatic device allows the audience to gain insight into the character's inner feelings and conflicts, often reflecting their true emotions or dilemmas. Soliloquies are commonly found in plays, particularly in the works of Shakespeare, where they serve to deepen character development and advance the plot.