Concept/Definition
An IDIOM is…
...a popular expression that has a meaning that is not immediately obvious.
Concept/Definition
A EUPHEMISM is…
...a specific type of idiom that is used to avoid saying an uncomfortable or sad truth.
EXAMPLE
"It's raining cats and dogs." (it's raining really hard.)
"I found myself between a rock and a hard place." (I was in a very difficult position.)
"A penny for your thoughts." (What are you thinking about?)
"He passed away." (euphemism for "he died")
(Want more examples? Click here for an awesome list of idioms and explanations.)