Oedema, also known as edema in the United States, or more properly lymphoedema, "is caused by the capilliary blood vessels passing out water into the surrounding tissues, and so increasing the intercellular fluid content." It manifests itself as a swelling of the penis after circumcision. It is believed to be more common after adolescent and adult circumcision than after circumcision of the newborn.
Circumcision severs certain veins that return blood to the heart. When return of the blood to the heart is impaired, then lymph builds up in the penis, and results in lymphoedema. It can take months for new lymph pathways to form so that the lymph can flow out of the penis.