means A notion is an idea, belief, or impulse — often a loose or half-formed one — that crosses your mind.

from From Latin notio, meaning 'a becoming acquainted, a concept,' built on notus, 'known' (the past participle of noscere, 'to know') — the same family that gives us 'notice' and 'note.' So a notion is, quite literally, a thing you've come to know, even if only barely. It entered English in the late 1500s as a term of philosophy and logic before relaxing into everyday vagueness.