A systemic disorder defined by a clonal expansion of mast cells that in most instances is indolent and nonmalignant. Generally involves bone marrow, skin, gastrointestinal (GI) mucosa, liver, and spleen. Classified as cutaneous mastocytosis with variants and four systemic forms: indolent, associated with concomitant hematologic disorder, aggressive, and mast cell leukemia. Mast cell sarcoma and extracutaneous mastocytomas are rare solid mast cell tumors.