As a Noun:
1. An idea which is good, reasonable or appropriate.
As a Verb or Adjective:
1. Of an idea, to be well thought out, clear, appropriate. English = (be) sensible, (be) reasonable.
2. To make decisions and judgements that are based on reason rather than on emotion. English = (be) rational.
3. [With stress] extremely well thought out. English = (be) brilliant.
1. Used alone in response to what someone has said or suggested to mean you think their idea is good, reasonable, appropriate or timely. English = 'What a good idea!', 'Now, that's a great idea!' and so on.
1. Used to introduce a sentence in which you express an idea you have, when you think that idea is very good, reasonable, appropriate or timely. English = 'I've got this great idea!', 'Hey! Why don't we...', 'Say, let's...' and so on.
2. Used to repeat someone else's suggestion favourably (ie, when you think it is a good idea too). English = 'That's a great idea....', 'What an idea!....'