Definitions of idea:

• noun:   the content of cognition; the main thing you are thinking about

  Example: "It was not a good idea"

• noun:   a personal view

  Example: "He has an idea that we don't like him"

• noun:   your intention; what you intend to do

  Example: "The idea of the game is to capture all the pieces"

• noun:   an approximate calculation of quantity or degree or worth

  Example: "A rough idea how long it would take"

• noun:   (music) melodic subject of a musical composition

  Example: "The accompanist picked up the idea and elaborated it"