Pitting edema: Observable swelling of body tissues due to fluid accumulation that may be demonstrated by applying pressure to the swollen area (such as by depressing the skin with a finger). If the pressing causes an indentation that persists for some time after the release of the pressure, the edema is referred to as pitting edema.
Any form of pressure, such as from the elastic in socks, can induce pitting with this type of edema. Edema can be caused by either systemic diseases (diseases affecting multiple organ systems) or by local conditions involving just the affected extremities. In systemic diseases that cause edema, the fluid accumulation is primarily due to the body's retention of too much salt (sodium chloride), causing the body to retain water.
Picture of Pitting Edema