The Collaborative International Dictionary of English v.0.48:

Compactness \Com*pact"ness\, n.
   The state or quality of being compact; close union of parts;
   density.
   [1913 Webster]

WordNet (r) 3.0 (2006):

compactness
    n 1: the spatial property of being crowded together [syn:
         concentration, density, denseness, tightness,
         compactness] [ant: dispersion, distribution]
    2: the consistency of a compact solid