idea

(noun): An abstract archetype of a given thing, compared to which real-life examples are seen as imperfect approximations; pure essence, as opposed to actual examples.
(noun): The conception of someone or something as representing a perfect example; an ideal.
(noun): The form or shape of something; a quintessential aspect or characteristic.
(noun): An image of an object that is formed in the mind or recalled by the memory.
(noun): More generally, any result of mental activity; a thought, a notion; a way of thinking.
(noun): A conception in the mind of something to be done; a plan for doing something, an intention.
(noun): A purposeful aim or goal; intent
(noun): A vague or fanciful notion; a feeling or hunch; an impression.
(noun): A musical theme or melodic subject.

The mere idea of you is enough to excite me.
I have an idea of how we might escape.
Yeah, that's the idea.
He had the wild idea that if he leant forward a little, he might be able to touch the mountain-top.