Sarcoidosis: A disease of unknown origin that causes small lumps (granulomas) due to chronic inflammation in body tissues. Sarcoidosis can appear in almost any body organ, but it most often starts in the lungs or lymph nodes. It can also affect the eyes, liver, and skin; and less often it affects the spleen, bones, joints, skeletal muscles, heart, and central nervous system. In the majority of cases, the granulomas clear up with or without treatment. In cases where the granulomas do not heal and disappear, the tissues tend to remain inflamed and become scarred.