Because we need to define terms to be able to speak about different phenomena, we differentiate between dream and daydream, but they are not entirely distinct. They bear much similarity. Both are internally generated experiences, both are often refer to things from our waking lives, both can be very emotional, both can be bizarre. However, by definition, daydreams happen during the day (or, more specifically, when we are awake), whereas dreams happen when we are asleep. Daydreams tend to be less weird and more obviously about things that are on our mind than dreams.
