Malignancy (from Latin male 'badly', and -gnus 'born') 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.

Tumours can be detected through the visualisation or sensation of a lump on the body. In cases where there is no obvious representation of a lump, a mammogram or an MRI test can be used to determine the presence of a tumour. In the case of an existing tumour, a biopsy would be then required to make a diagnosis as this detects if the tumour is malignant or benign. This involves examination of a small sample of the tissue in a laboratory. If detected as a malignant tumour, treatment would be necessary. Treatment during early stages is most effective. Forms of treatment include chemotherapy, surgery, photoradiation and hyperthermia, amongst various others.