Story

STORY, as defined in the UBC, is that portion of a building included between the upper surface of any floor and the upper surface of the floor next above, except that the topmost story shall be that portion of a building included between the upper surface of the topmost floor and the ceiling or roof above. If the finished floor level directly above a usable or unused under-floor space is more than 6 feet (1829 mm) above grade as defined herein for more than 50 percent of the total perimeter or is more than 12 feet (3658 mm) above grade as defined herein at any point, such usable or unused under-floor space shall be considered as a story.
This can be very important because the code is quite specific about number of stories permitted. For smaller buildings, three stories is an important cut off point. Mezzanines are not stories, therefore a two story building with mezzanines on both stories-- making it four levels-- is still a two story building as long as the mezzanines comply with all requirements