idealist

1. One whose conduct is influenced by ideals that often conflict with practical considerations.
2. One who is unrealistic and impractical; a visionary.
3. An artist or writer whose work is imbued with idealism.
4. An adherent of any system of philosophical idealism.

noun

A person inclined to be imaginative or idealistic but impractical: dreamer, utopian, visionary. See ability/inability, hope/despair.

Definition: person who holds fancies in mind
Antonyms: cynic, defeatist, pragmatist, realist

The noun has one meaning:

Meaning #1: someone guided more by ideals than by practical considerations
  Synonym: dreamer