Tumor -- Tissue composed of cells that deviate from normal program of cell division and differentiation.

Oncogenesis -- Process of initiation of tumors (cancer) in an organism.

Benign Tumor -- Tumor cells remain together in a single mass and do not invade or disrupt surrounding tissues.

Malignant Tumor -- Tumor cells invade and disrupt surrounding tissues (and are diagnosed as cancer).

Metastases -- Spread of malignant tumor cells throughout the body (typically through the blood and lymphatic system).

Mutation -- errors in DNA sequence at the time of cell division.

Apoptosis -- The mechanism by which old or damaged cells normally self-destruct.

Carcinogens -- Chemicals in our surroundings which cause cancer.