The word cancer has its origin from Latin word Cancrum (Greek - Karakinos) which means crab. Various Types of tumours have also been described in Sushruta Samhita.
It is defined as in abnormal growth of sense in any tissue or organ of body. It has the probation to spread and grow in other part of the body. The cancer cells preys on host cells and continue to grow indefinitely competing with normal cells for nutrition.
Most debatable question of 20th century is what causes cancer. After long period of confusion it is finally established that it is caused by mutation in both regulatory gens and tumour suppressor gens. Normal cell division (mitosis) in our body is highly regulated mechanism controlled by gens (made of DNA) through both regulatory pathways. A log exposure two carcinogens damage DNA and induce nutation in growth regulatory gens including oncogenes and tumour suppressor gens leading to loss of control over normal cell division. These mutated cells proliferate indiscriminately forming a mass cold neoplasm or malignant tumour or in simple words cancer. With passage of time cancer cells further mutated.
More dangerous characteristics such as ability to invade and move into adjoining tissue flow through live and blood vessels lodge and grow into other parts of the body. And form colonies this process is called metastasis. Cancer cells can even create their own blood vessel (tumour angiogenesis) for their nutrition and become immortal by gaining ability of limitless replication.
By the time cancer detected they have already added many Mutations.