What characteristic defines a malignant tumor?

A malignant tumor is specifically characterized by its ability to metastasize, which means it can spread to distant organs and tissues in the body. This propensity for invasion and spread distinguishes malignant tumors from benign tumors, which typically remain localized and do not invade nearby tissues or metastasize.

The capability of metastasis involves the tumor's cells breaking away from the original tumor mass, entering the bloodstream or lymphatic system, and establishing new tumors in other organs. This aggressive nature is a defining trait of malignancy and is often associated with more serious health implications, requiring different treatment approaches compared to benign growths.

The other options relate to features typically associated with benign tumors or tumors in general. For instance, a localized tumor would imply that it is benign and does not have the potential to spread. A tumor surrounded by a capsule is also typically benign, indicating that it is contained and less likely to invade surrounding tissues. Finally, classifying a tumor as benign directly contradicts the characteristics of a malignant tumor. Understanding these distinctions helps in differentiating the nature of tumors in the medical field.