Final answer:
A lengthy speech by a character who is alone on stage revealing their thoughts is called a soliloquy, which allows the audience to hear the character's innermost reflections.
Step-by-step explanation:
A lengthy speech in which a character, alone onstage, expresses their thoughts aloud is known as a soliloquy. This kind of speech is different from a monologue or dialogue because it is performed by a character who is alone, revealing their innermost thoughts directly to the audience without any other character's presence. It is a powerful dramatic device used extensively in theatrical plays such as those by William Shakespeare. A well-known example is the "To be, or not to be..." soliloquy from Shakespeare's Hamlet, where the character Hamlet contemplates existence.