Definition of idea:
the content of cognition; the main thing you are thinking about; "it was not a good idea"; "the thought never entered my mind"
your intention; what you intend to do; "he had in mind to see his old teacher"; "the idea of the game is to capture all the pieces"
a personal view; "he has an idea that we don't like him"
an approximate calculation of quantity or degree or worth; "an estimate of what it would cost"; "a rough idea how long it would take"
(music) melodic subject of a musical composition; "the theme is announced in the first measures"; "the accompanist picked up the idea and elaborated it"