The Collaborative International Dictionary of English v.0.48:

Diapedesis \Di`a*pe*de"sis\, n. [NL., fr. Gr. ? a leaping or
   oozing through, fr. ? to leap through; dia` through + ? to
   leap.] (Med.)
   The passage of the corpuscular elements of the blood from the
   blood vessels into the surrounding tissues, without rupture
   of the walls of the blood vessels.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diapedesis
    n 1: passage of blood cells (especially white blood cells)
         through intact capillary walls and into the surrounding
         tissue