Idea (noun)

1. A thought or suggestion as to a possible course of action.
2. A mental picture of something you have seen, heard, or experienced.
3. An opinion or belief, especially one not based on proof or evidence.
4. A plan or concept for something.

Origin:

Late middle english from latin idea 'form, shape, concept'

Examples:

1. She had the idea of starting her own business.
2. I have no idea what you are talking about.
3. It was just an idea and we were exploring possibilities.
4. He had a vague idea of what he wanted to do in life.
5. The idea behind the project was to help the less fortunate.