1 : a serious disease caused by cells that are not normal and that can spread to one or many parts of the body [noncount] ▪ breast/lung cancer ▪ He was diagnosed with cancer. ▪ She learned that she has cancer. ▪ Eating certain foods may help reduce the risk of cancer. [count] ▪ Advanced cancers are more difficult to treat. —often used before another noun ▪ cancer patients ▪ cancer prevention ▪ cancer treatments
2 [count] : something bad or dangerous that causes other bad things to happen —usually singular ▪ Her jealousy was a cancer that slowly destroyed her marriage. ▪ Drugs and violence have become a cancer in the city.
3 Cancer a [noncount] : the fourth sign of the zodiac that comes between Gemini and Leo and has a crab as its symbol —see picture at zodiac b [count] : a person born under the sign of Cancer : a person born between June 22nd and July 22nd ▪ I'm a Taurus, but my best friend is a Cancer.