To fantasize is to imagine things that are not true and sometimes not possible. We fantasize about things we wish were real.
Have you ever daydreamed that you're a king, or a superstar in the NBA, or able to fly? Then you've been fantasizing. Fantasizing is something everyone does, though we all have different fantasies. You can fantasize about things that are possible or impossible, including being the President, riding a unicorn, or being the President of unicorns. People also fantasize about things they probably shouldn't do, like romancing the neighbor's spouse. You can fantasize about anything you can imagine.