WordNet (r) 3.0 (2006):

hypertonicity
    n 1: (of a solution) the extent to which a solution has a higher
         osmotic pressure than some other [ant: hypotonicity]
    2: (of muscular tissue) the state of being hypertonic [syn:
       hypertonia, hypertonus, hypertonicity] [ant:
       hypotonia, hypotonicity, hypotonus]