What term describes a short speech expressing a character's true thoughts in a play?

The term that correctly describes a short speech expressing a character's true thoughts, typically given when the character is alone on stage, is a soliloquy. A soliloquy allows the audience to gain insight into the character's inner feelings and motivations, often revealing secrets or personal dilemmas that the character does not share with others in the play.

While a monologue is also a long speech delivered by one character, it may be directed to other characters and does not necessarily convey the character's private thoughts. An aside, on the other hand, is a brief remark made by a character to the audience or another character that is not intended to be heard by all characters on stage, but it does not capture the depth of thought typically found in a soliloquy. Dialogue involves conversations between characters and does not single out an individual character's inner thoughts. Therefore, the best fit for the description given is a soliloquy.