Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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