I usually think of a dream as a path of storyline, or several paths that somehow meld into eachother, or in other words: a dream is whatever happens between periods of "blackness" or just a general void or no dreams. For example, I may have a dream that I'm interacting with characters from a book, then I could just be sitting in my kitchen, then I could be somewhere physically impossible, then (continues...) until I get to where there is no "imagery" or other sensory input.