What is Idea ?
Idea is (noun) 1. something which you think of to have an idea that to think that I have an idea that the buses don’t run on Sundays. to have no idea, not to have the faintest idea not to know Where’s your brother? – I’ve no idea or I haven’t the faintest idea. I had no idea it was as late as that. 2. a plan which you make in your mind Some of his ideas were really original. I’ve had an idea – let’s all go for a picnic! That’s a good idea!
source: Easier English, Student Dictionary Upper Intermediate Level