Mental apprehension of whatever may be known or imagined; an idea; a conception; more properly, a general or universal conception, as distinguishable or definable by marks or notæ. [1913 Webster]
"What hath been generally agreed on, I content myself to assume under the notion of principles." [1913 Webster]

"Few agree in their notions about these words." [1913 Webster]

"That notion of hunger, cold, sound, color, thought, wish, or fear which is in the mind, is called the “idea” of hunger, cold, etc." [1913 Webster]

"Notion, again, signifies either the act of apprehending, signalizing, that is, the remarking or taking note of, the various notes, marks, or characters of an object which its qualities afford, or the result of that act." [1913 Webster]
A sentiment; an opinion. [1913 Webster]
"The extravagant notion they entertain of themselves." [1913 Webster]

"A perverse will easily collects together a system of notions to justify itself in its obliquity." [1913 Webster]
Sense; mind. Shak. [1913 Webster]
An invention; an ingenious device; a knickknack; as, Yankee notions. [1913 Webster]
Inclination; intention; disposition; as, I have a notion to do it. [1913 Webster]
Miscellaneous small objects; sundries; -- usually referring to articles displayed together for sale. [PJC]

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OXFORD DICTIONARY

Notion, n.
1 a a concept or idea; a conception (it was an absurd notion). b an opinion (has the notion that people are honest). c a vague view or understanding (have no notion what you mean).
2 an inclination, impulse, or intention (has no notion of conforming).
3 (in pl.) small, useful articles, esp. haberdashery.

Etymology

L notio idea f. notus past part. of noscere know

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THESAURUS

Notion

apprehension, assumption, attitude, automatic response, bare suggestion, bee, blind impulse, boutade, brain wave, brainstorm, capriccio, caprice, climate of opinion, clue, common belief, community sentiment, conceit, concept, conception, conclusion, consensus gentium, consideration, crank, craze, crazy idea, crotchet, cue, drive, estimate, estimation, ethos, eye, fad, fancy, fantastic notion, fantasy, feeling, flash, fleeting impulse, flimflam, fool notion, freak, freakish inspiration, general belief, general idea, gut response, half an idea, harebrained idea, hazy idea, hint, humor, idea, image, imago, impression, impulse, inclination, indication, inkling, inspiration, instinct, intellection, intellectual object, intimation, involuntary impulse, judgment, kink, lights, maggot, megrim, memory-trace, mental image, mental impression, mental picture, mere notion, mind, mystique, natural impulse, observation, opinion, passing fancy, perception, personal judgment, point of view, popular belief, position, posture, presumption, prevailing belief, public belief, public opinion, quick hunch, quirk, reaction, recept, reflection, reflex, representation, sentiment, sight, sneaking suspicion, stance, sudden thought, suggestion, supposition, suspicion, telltale, theory, thinking, thought, toy, urge, vagary, vague idea, view, way of thinking, whim, whim-wham, whimsy, wind

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ROGET THESAURUS

Notion

Idea

N idea, notion, conception, thought, apprehension, impression, perception, image, eidolon, sentiment, reflection, observation, consideration, abstract idea, archetype, formative notion, guiding conception, organizing conception, image in the mind, regulative principle, view, theory, conceit, fancy, phantasy, point of view, field of view.

Supposition

N supposition, assumption, assumed position, postulation, condition, presupposition, hypothesis, blue sky hypothesis, postulate, postulatum, theory, thesis, theorem, data, proposition, position, proposal, presumption, divination, conjecture, guess, guesswork, speculation, rough guess, shot, shot in the dark, conjecturality, surmise, suspicion, sneaking suspicion, estimate, approximation (nearness), inkling, suggestion, hint, intimation, notion, impression, bare supposition, vague supposition, loose supposition, loose suggestion, association of ideas, (analogy), metonym, metonymy, simile (metaphor), conceit, idea, thought, original idea, invention (imagination), supposing, given, mooted, postulatory, assumed, supposititious, suppositive, suppositious, gratuitous, speculative, conjectural, hypothetical, theoretical, academic, supposable, presumptive, putative, suppositional, suggestive, allusive, if, if so be, an, on the supposition, ex hypothesi, in the case, in the event of, quasi, as if, provided, perhaps, for aught one knows.

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