Mastitis is a painful swelling and infection of the breast in a woman, or of the udder in an animal that gives milk, such as a cow. The part becomes hot, sore, and often red; feeding or milking can hurt. It is a medical word. Mothers can get it while breastfeeding; farmers worry about it in dairy animals because the milk and the animal both suffer. Though actually, people sometimes say 'breast infection' in ordinary talk; mastitis is the name a doctor or vet will write. It is not a tumour by itself, and it is not the dog mastiff — only the first letters match.