I would define a landscape as a piece of artwork that includes a foreground and background, and covers a substantial area of space. I don't think there has to be a lot of limitation on the subjects in the art, and there doesn't necessarily have to be a specific subject(s). In my landscape sketches/painting so far, I've tried to include these elements--specifically a foreground and background, because I think that it adds a lot to the element of distance and space in a landscape. Especially in my painting (probably because its easier to do with color), I tried to pay a lot of attention to the natural light in the landscape and how the tones of certain objects were changed significantly just by the addition of a ray of light hitting it, which I also think helped to break up all the green hues in the painting. Without the light causing different shades and variations of yellows in the painting, it would be pretty dull and just simple and green, so the light helps to communicate the vibrancy of nature's colors into the art.