>An impossibility
>A dream or fantasy
>Something that is unlikely to happen / won’t work
> A hope, plan or idea that will most likely fail
Similar meanings are conveyed through the idioms when pigs fly and when hell freezes over
Example
His plans of becoming an astronaut are a pipe dream, he should be more realistic.
As she bent her head to receive the gold medal, she realised competing for her country in the Olympics was no longer a pipe dream.