i·de·a
n. 
Something, such as a thought or conception, that potentially or actually exists in the mind as a product of mental activity. 
An opinion, conviction, or principle: has some strange political ideas. 
A plan, scheme, or method. 
The gist of a specific situation; significance: The idea is to finish the project under budget. 
A notion; a fancy. 
Music A theme or motif. 
Philosophy 
In the philosophy of Plato, an archetype of which a corresponding being in phenomenal reality is an imperfect replica. 
In the philosophy of Kant, a concept of reason that is transcendent but nonempirical. 
In the philosophy of Hegel, absolute truth; the complete and ultimate product of reason.