Any of a group of malignant neoplasms derived from cells endogenous to lymphoid tissue. Lymphomas are grouped into two major categories: Hodgkin's disease and non-Hodgkin's lymphomas. Lymphomas usually originate in the lymph nodes located throughout the body, but they can arise from lymphoid tissue that does not form distinct nodes, such as that in the gastrointestinal tract or lung. Determination of the specific variety of Hodgkin's disease or non-Hodgkin's lymphoma was formerly based on the appearance of the cells when examined under a light microscope. Identification now relies on the nature of the cells with respect to certain substances (antigens) that they have on their surface or within their cytoplasm. For clinical purposes, lymphomas are categorized into three grades, low, intermediate, and high, with low-grade lymphomas having the best prognosis.