idealism (n.)

early 15c., "pertaining to an archetype or model," from Late Latin idealis "existing in idea," from Latin idea in the Platonic sense (see idea). Senses "conceived as perfect; existing only in idea," are from 1610s.

word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.

"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).