When you vaticinate, you tell the future. A carnival fortune teller might vaticinate that you'll travel the world and have seven children.
When you make a prediction about what will happen, whether it's a guess or a prophecy, you vaticinate. You might vaticinate about the next day's weather or what will happen on your favorite television show in the upcoming season. A magician might vaticinate what card you'll pull out of a deck. The Latin root word of vaticinate is vates, which means "prophet."