We do not easily arrive at a complete and simple definition of lymphoma unless we begin with the basics and work our way into the more complex aspects of this disease, and even then there is no certainty that it will be understood.
For starters, lymphoma is a malignancy that occurs in lymphocytes, the white blood cells that patrol our body in a network of vessels that make up part of the lymphatic system. This network traverses the entire body, meaning that lymphoma can develop nearly anywhere in the body, provided the cells are there.
The lymphatic system is part of our immune system and it includes primary organs and secondary organs. The lymph system's primary organs are the thymus and the bone marrow. The lymph system's secondary organs include the spleen and the many hundreds of lymph nodes located throughout the body. The lymphocytes traverse our body, passing through these organs at various stages of their life cycles.
Part of that life cycle involves beginning a process that allows the cell, in a manner of speaking, to commit suicide, known as apoptosis. This is an important process, and is necessary for our good health that we continue to produce lymphocytes and that they continue to patrol our bodies looking for pathogens and that they eventually die.
Among the many reasons this is important is because when a lymphocyte refuses to die, there is a big problem brewing: a lymphocyte that won't die may be able to undergo mitosis—divide and multiply, in other words. And when a cell which has mutated to the point that it can dodge the pre-programmed cell suicide, and when that cell can safely then divide, we have an uncontrolled proliferation of malignant cells—a very basic, fundamental definition of cancer. in this case, cancer occurring in lymph cells.
To put it briefly, in lymphoma, cancerous lymphocytes begin to proliferate, rather than die. Thus, in a definition of lymphoma, it is considered a lymphoproliferative disorder.
When this little mutation emerges from other cells in the body—breast tissue cells, skin cells, brain cells—it is a cancer of those cells—breast, skin, brain cancers.
Lymphoma subtypes—of which there are literally dozens and dozens—are generally named in part by how they look. When we see subtypes like 'anaplastic large cell lymphoma' this merely means that under the microscope, the diseased cells are misshapen (anaplastic) and bigger in size than healthy cells (large).
The greater categorization for lymphomas is beyond the scope of this entry, and in fact is in a near-constant flux, as researchers learn more and more about what makes lymphomas that form in one place the same or different from lymphomas that form in another place.