You might be convinced that one day you'll buy a private island and turn it into a cat sanctuary — but your parents will probably call it a pipe dream, or an unrealistic, wild scheme.
Impossible dreams and plans are pipe dreams. Imagining that life will be like a Disney movie is one kind of pipe dream, and fantasizing about becoming a famous rock star is another kind of pipe dream. Some pipe dreams do come true, but most of them won't ever be more than a crazy idea. This term dates from the 1870s, and it originally referred to the strange, hallucinatory dreams that opium smokers had.