A child fantasizes about being a superhero.
When you think about the nice house you'd buy if you won the lottery, this is an example of when you fantasize.
Origin of fantasizefantas(y) + -ize
verbfan·ta·sized, fan·ta·siz·ing, fan·ta·siz·es
(third-person singular simple present fantasizes, present participle fantasizing, simple past and past participle fantasized)