Pyoderma gangrenosum: An ulcerating condition of skin that results in heaped borders with a typical appearance. Pyoderma gangrenosum appears to be mediated by the immune system, but the exact cause is unknown. The lesion usually begins as a soft nodule on the skin that then ulcerates. The ulcer enlarges, and the skin at the edge is purple-red. The ulcers can become quite large. Pyoderma gangrenosum is associated with several other diseases, including ulcerative colitis, Crohn's disease, rheumatoid arthritis, leukemia, and cryoglobulinemia. Pyoderma gangrenosum is usually responsive to corticosteroids.