"one who represents things in an ideal form," 1829, from ideal + -ist. Earlier (1796) in a philosophical sense "one who believes reality consists only in (Platonic) ideals" (see idealism).
It seems even incredible, that any Idealist in any age could forget himself so far as to run his head against a post, merely because he found in his system, that no external world does exist, and that therefore nothing could be without to hurt him. [F.A. Nitsch, "A General and Introductory View of Professor Kant's Principles," 1796]
Earlier still, "one who holds doctrines of philosophical idealism" (1701).