I'll expand on that, not because you're wrong, but because I'm bored and feel like it. A monologue is when a character in the play reveals parts of his/her emotions, intent, and personality to the audience. While sometimes there could be other characters witnessing the monologue, it is usually for the benefit of the audience members. Shakespeare would often use monologues to reveal the personalities of his characters. For example, the Queen Mab speech in Romeo and Juliet shows exactly who Mercutio is. The example you gave of a king addressing his people could possibly be a monologue. Staying on the theme of Shakespeare, a good example of a monologue taking the form of an address like the one you mentioned would be Marc Antony's famous monologue in Julius Caesar, where he speaks to a large crowd. This speech however, isn't considered a monologue because it is a long speech, it's a monologue because of what it reveals about Marc Antony.