a circumscribed inflammatory and often suppurating lesion on the skin or an internal mucous surface resulting in necrosis of tissue
"After ignoring the persistent burning pain for weeks, he finally visited a doctor who confirmed that his chronic stress had caused a painful ulcer to form inside his stomach."
An open sore of the skin, eyes or mucous membrane, often caused by an initial abrasion and generally maintained by an inflammation and/or an infection.
"After the rough edge on his shoe scraped against his heel, a painful ulcer formed on the sensitive skin that refused to heal despite weeks of bandaging."