One who thinks
that things in the world
can be made perfect
, can match their ideal
implementation, and wants things to be that way.
Youth tend to be idealists, thinking it's possible to rid the political system of the corrupt, it's possible to get industry to avoid harming the environment. Then they find out there are too many selfish idiots who don't care for there to be any chance for the world to be nice and that the world sucks, always has, and always will.
What is odd is that I've seen people who use the term idealist as an insult, as if it is somehow a bad thing to believe that the world can be made better, and that we should, for some reason, prefer the world as it currently is.