a basement is defined in the building codes (and I assume the NEC is not going to argue with the building codes over this definition since they have not bothered creating a different definition)-
Uniform Bldg. Code -
"Basement is any floor level below the first story in a bldg., except that a floor level in a bldg. having only one floor level shall be classified as a basement unless such floor level qualifies as a first story as defined herein." . . .. now is that confusing or what? . . . however it does go on (in definitions of story, first story) to define what constitutes the 1st floor...I'm sure the bldg. codes in your area will define basements/1st story.