The Collaborative International Dictionary of English v.0.48:

Higher National Diploma \Higher National Diploma\ n. (Education)
   A certificate awarded for completing a course of vocational
   education beyond secondary school, preparing the student for
   a career in business or certain practical arts. It is a term
   used in the United Kingdom. [United Kingdom]
   [PJC]

WordNet (r) 3.0 (2006):

Higher National Diploma
    n 1: a diploma given for vocational training that prepares the
         student for a career in a particular area; good students
         may progress to a course leading to a degree [syn: Higher
         National Diploma, HND]