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1829; see idea + -ation. Related: Ideational.
As we say Sensation, we might say also, Ideation; it would be a very useful word; and there is no objection to it, except the pedantic habit of decrying a new term. [James Mill, "Analysis of the Phenomena of the Human Mind," London, 1829]
ideation i·de·a·tion (ī'dē-ā'shən)n. The formation of ideas or mental images.