A lucid dream is a dream in which the dreamer realises that they are dreaming. The important fact, which we must not lose sight of, is that most dreams are not lucid, that is, the dreamer does not realise that they are dreaming.
    There exist special techniques and even special devices that can be used to induce dream lucidity. Techniques include noticing specific scenes that tend to occur in dreams, and mentally rehearsing the realisation that such a scene implies that one is in a dream. If this fails, there are red lights that flash into one's eyes when REM sleep occurs. As is often the case, external sensations can be incorporated into a dream, and one can rehearse using red light as a cue to realise that one is dreaming.
    Both these techniques attempt to work around a major deficit in our awareness that occurs during dreaming. If a dream-like situation occurred when we were awake, we would very quickly realise that something strange was going on, and would very likely come to the conclusion that we might be dreaming. The mental faculties that would enable us to recognise such "strangeness" are apparently switched off when we are dreaming. It may indeed be the locus ceruleus which is the brain's "strangeness" detector, as the locus ceruleus is known to react to novel stimuli when the brain is awake, and it is known to be deactivated during REM sleep, which is when most dreaming occurs.