This, now, is tricky because soliloquy is not simply a verse feature. It, however, can be in verse as well as prose forms. This a feature in plays. Soliloquy is when an actor speaks to himself on stage. Soliloquy originates from Latin words solus to mean alone and loqui to mean speak and so its literal meaning would be to speak alone.
To be or not to be from Shakespeare's The Tragedy of Hamlet is one of the most popular soliloquies, we know, today. And during this soliloquy, the actor is alone on stage, speaking either to himself or in some productions to the audience.
This is a very effective literary device used by playwrights to communicate the inner emotions and complexities of the play's character(s) to audiences.