| the passage of blood cells, esp. leukocytes, through the unruptured walls of the capillaries into the tissues. |
dēsis a leaping through, equiv. to diapēdē- (verbid s. of diapēdân to leap through) + -sis -sis 
diapedesis di·a·pe·de·sis (dī'ə-pĭ-dē'sĭs)
n.
The movement or passage of blood cells, especially white blood cells, through intact capillary walls into surrounding body tissue. Also called migration.