When you guess about how something is going to come out (or how it happened), that's speculation. You're making an educated guess.
When people predict who will win a football game, an Oscar, or an election, it's speculation: people are looking at the facts and making their best guess. Just about anything you say about the future is speculation, because no one knows what will happen. The word is used in the stock market for such financial dealings as "buying on spec," a risky way to make money. Sometimes, this word means something close to meditation — pondering something deeply.