To fantasize is defined as to imagine or daydream.
(verb)When you think about the nice house you'd buy if you won the lottery, this is an example of when you fantasize.
See fantasize in Webster's New World College Dictionary
transitive verb fantasized, fantasizing
Origin: fantas(y) + -ize
intransitive verb
See fantasize in American Heritage Dictionary 4
verb fan·ta·sized, fan·ta·siz·ing, fan·ta·siz·es verb, transitive
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