A monologue is a speech for a single voice that is not part of a conversation with other characters.
In a poem, a monologue is called a dramatic monologue. This type of poem is written from a speaker's point of view and it seems as if the speaker is addressing the reader of the poem.
In drama, the most usual type of monologue is the interior monologue. A soliloquy is a common form of interior monologue. This allows the audience to have privileged access to the character's inner thoughts and feelings about a particular situation or circumstance. Interior monologues are a useful way of developing dramatic irony.