Of course, its probably just as common to be thinking during your dreams than it is to think when you're awake. The difference is, i think, when your'e dreaming you usually take a back seat and dont realise that you are actually there and aware. Lucidity is merely making this realisation. When you are in a dream state and are referencing (thinkin, speaking of) your body which is asleep in the waking world, you are generally lucid. Doesnt matter for how long.

Lucidy is not thinking inside a dream. It is the recognition and affirmation that you are in a dream, while your body lies there sleeping. It is a very simple definition, actually, and will become clearer to you as you explore the dream world.