Dictionary:

utopian

  (yū-tō'pē-ən)

1. often Utopian Of, relating to, describing or having the characteristics of a Utopia: a Utopian island; Utopian novels.
2. 1. Excellent or ideal but impracticable; visionary: a utopian scheme for equalizing wealth.
   2. Proposing impracticably ideal schemes.

A zealous but impractical reformer of human society.

adjective

1. Not compatible with reality: idealistic, quixotic, romantic, starry-eyed, unrealistic, visionary. See hope/despair, real/imaginary.
2. Showing a tendency to envision things in perfect but unrealistic form: idealistic, visionary. See hope/despair, real/imaginary.

noun

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

Definition: imaginary, ideal
Antonyms: real

The adjective has one meaning:

Meaning #1: being the best possible; characterized by ideal perfection
  Antonym: dystopian (meaning #1)