idealist

First recorded in 1695–1705; ideal + -ist

Originally the term idealist applied to followers of any philosophical school of thought (often called idealism ) which emphasized the role of ideas in explaining reality. Over time, the meaning of the term broadened to include first anyone strongly guided by an ideal, and then those who believed, often unrealistically, in something that might be unattainable—which is probably the most common use of the word today.
Ideal comes from the Late Latin word ideālis, which means “existing as an idea or archetype.” The earliest recorded use of idealist in English occurs in 1701 in philosopher John Norris’s Essay toward the Theory of the Ideal or Intelligible World, in which he references the Greek philosopher Plato’s “theory of forms.” Plato had posited that everything we perceive is actually a representation of ideal things, but not the things themselves. Idealism gained popularity in various guises in the 18th-century works of philosophers such as Berkeley, Kant, and Hegel.
By the start of the 19th century, the meaning of idealist broadened to describe artists or writers who treated subjects with imagination, in contrast to a naturalist or realist, who depicted a real-world atmosphere in their art. A few decades later, the term was applied to visionaries, and soon after to people who were so imbued with an ideal that they failed to see the world for what it is. Today, the word can be a two-edged sword: if a person calls herself an idealist she very likely means it positively, as in the pursuit of a higher good. However, if somebody else calls her an idealist, that person can mean that she is impractical or naive.

—Idealist.org: a job-search website that connects people who want to work or volunteer their services with not-for-profit organizations.