Pityriasis rosea: A common mild rash of unknown origin that can appear on a person of any age (most commonly at 10-35 years of age). It may last from several weeks to a few months, often begins with a "herald" patch, a large single pink patch on the chest or back and, within a week or so, more pink papules appear on the torso, arms and legs. There may be itching, especially when overheated. Treatment may include medications for the itching and soothing lotions or lubricants, and lukewarm rather than hot baths. There is no specific therapy for this self-limited process.