Malignant most commonly refers to any growth or tumor which is cancerous. Malignant growths have the ability to spread to other areas of the body. By definition, any cancer is considered malignant.
Malignant is sometimes used to refer to any condition, cancerous or not, which has the potential for serious health problems. This second definition could obviously encompass nearly any medical problem.
The opposite of malignant is benign. With regard to the prostate, prostate cancer is malignant, while BPH (enlarged prostate) is considered benign because it is not cancer and cannot spread distantly to other parts of the body.