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. Ideagenous "generating ideas" (1839) is a word from early psychology, apparently coined by Dr. Thomas Laycock, house surgeon to York County Hospital ["Edinburgh Medical and Surgical Journal," vol. lii].
"(hypothetical) perfect person, thing, or state," 1796, in a translation of Kant, from ideal (adj.). Hence "standard or model of perfection" (1849).