Mezzanine

A mezzanine is an intermediate floor between two main floors, usually open on one side to the space below. In homes the same idea shows up as a loft or a balcony overlooking a two-story room.

Under ANSI Z765, the floor of a finished loft counts as gross living area if it meets the ceiling height rules and is reached by a permanent stair. The open air it looks down into does not count, because there is no floor there.

A loft reached only by a ladder is left out, no matter how well it is finished.

Further reading: Mezzanine on Wikipedia.