Cancer n. : A genus of decapod Crustacea, including some of the most common shore crabs of Europe and North America, as the rock crab, Jonah crab, etc. See Crab.

Cancer n. : The fourth of the twelve signs of the zodiac. The first point is the northern limit of the sun's course in summer; hence, the sign of the summer solstice. See Tropic.

Cancer n. : A northern constellation between Gemini and Leo.

Cancer n. : Formerly, any malignant growth, esp. one attended with great pain and ulceration, with cachexia and progressive emaciation. It was so called, perhaps, from the great veins which surround it, compared by the ancients to the claws of a crab. The term is now restricted to such a growth made up of aggregations of epithelial cells, either without support or embedded in the meshes of a trabecular framework.

Example:
How can Cancer develop in really healthy people taking good care of themselves?

Taxonomy
cancer, cells, tumor, treatment, mutations, cancer cells, tissue, risk, genes, surgery, Tumor suppressor genes, patients, malignancies, cancer patients, disease, Treatment trials, breast cancer, oncogenes.

Defination from the web:
cancer, including its nomenclature, symptoms, treatments, prognosis, and prevention.