Here is a good definition which I really liked comes from the website of Fundación Ciencia & Vida, Santiago, Chile.
"Cancer is a group of diseases that share a commonality of uncontrolled growth of affected cells leading to tumors, metastases and, in many cases, death.
There is presently no definitive cure for cancer and the available treatments are very aggressive, with devastating effects on normal as well as tumor tissue" -- Luis O. Burzio, PhD
The first sentence with 25 words succinctly defines cancer by summing up three important elements:
- Cancer is a group of diseases. It can be a any of the 100 types of cancers effecting all kinds of organs in the body.
- They all share something, ie, the cells have lost normal cues and controls to cell division and differentiation.
- The consequence of uncontrolled proliferation varies with cancer type, but they all affect normal organ functions, disrupt normal physiology and may lead to organ failure and death.
Here are definitions of cancer from National Cancer Institute, American Cancer Society and Oxford Dictionary.