WordNet (r) 3.0 (2006):

waistline
    n 1: the narrowing of the body between the ribs and hips [syn:
         waist, waistline]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "waistline":
   center, core, diameter, diaphragm, equator, heart, interior,
   kernel, mean, median, middle, midmost, midriff, midst, nucleus,
   thick, thick of things, waist, zone