idea (nom)
Sens 1 : a plan, etc. formed by thinking; thought. [source : D'après OAL]
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Sens 2 : a mental impression. [source : D'après OAL]
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Sens 3 : an opinion or a belief about something. [source : OAAD]
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Sens 4 : "the idea": the aim or purpose of something. [source : OAAD]
ideal (adj)
Sens 1 : satisfying one's conception of what is perfect; most suitable. [source : OD]
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Sens 2 : existing only in the imagination or as an idea; unrealistic and so not
likely to be achieved. [source : OAL]
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Sens 1 : a conception of something in its absolute perfection. [source : AHD]
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Sens 2 : a principle about what is morally right or a perfect standard that you hope to achieve. [source : LG]