Malignancy () is the tendency of a medical condition to become progressively worse. Malignancy is most familiar as a characterization of cancer. A malignant tumor contrasts with a non-cancerous benign tumor in that a malignancy is not self-limited in its growth, is capable of invading into adjacent tissues, and may be capable of spreading to distant tissues. A benign tumor has none of those properties. Malignancy in cancers is characterized by anaplasia, invasiveness, and metastasis. Malignant tumors are also characterized by genome instability, so that cancers, as assessed by whole genome sequencing, frequently have between 10,000 and 100,000 mutations in their entire genomes. Cancers usually show tumour heterogeneity, containing multiple subclones. They also frequently have reduced expression of DNA repair enzymes due to epigenetic methylation of DNA repair genes or altered microRNAs that control DNA repair gene expression. Uses of “malignant” in oncology: Malignancy, malignant neoplasm and malignant tumor are synonymous with cancer Malignant ascites Malignant transformation Non-oncologic disorders referred to as “malignant”: Malignant hypertension Malignant hyperthermia Malignant otitis externa Malignant tertian malaria (malaria caused specifically by Plasmodium falciparum) Neuroleptic malignant syndrome