malignant

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Tending to metastasize.
• adjective Virulent or threatening to life.
• adjective Having or showing ill will; malicious.

from The Century Dictionary.

• Disposed to inflict suffering or cause distress; having extreme malevolence or enmity; virulently hostile; malicious: as, a malignant heart.
• Virulently harmful or mischievous; threatening great danger; pernicious in influence or effect.
• Specifically
• In astrology, threatening to fortune or life; fateful: as, the malignant aspect of the stars.
• In pathology, virulent; tending; to produce death; threatening a fatal issue: as, a malignant ulcer; a malignant fever; malignant pustule or scarlet fever.
• Extremely heinous: as, the malignant nature of sin.
• Synonyms Malevolent, bitter, rancorous, spiteful, malign. See animosity.
• noun A person of extreme enmity or evil intentions; an ill-affected person.
• noun Specifically, in English history, one of the adherents of Charles I. and his son Charles II. during the civil war; a Royalist; a Cavalier: so called by the Roundheads, the opposite party.

from the GNU version of the Collaborative International Dictionary of English.

• noun A man of extreme enmity or evil intentions.
• noun (Eng. Hist.) One of the adherents of Charles I. or Charles II.; -- so called by the opposite party.
• adjective Disposed to do harm, inflict suffering, or cause distress; actuated by extreme malevolence or enmity; virulently inimical; bent on evil; malicious.
• adjective Characterized or caused by evil intentions; pernicious.
• adjective (Med.) Tending to produce death; threatening a fatal issue; virulent.
• adjective (Med.) a very contagious disease produced by infection of subcutaneous tissues with the bacterium Bacillus anthracis. It is transmitted to man from animals and is characterized by the formation, at the point of reception of the infection, of a vesicle or pustule which first enlarges and then breaks down into an unhealthy ulcer. It is marked by profound exhaustion and often fatal. The disease in animals is called charbon; in man it is called cutaneous anthrax, and formerly was sometimes called simply anthrax.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Harmful, malevolent, injurious.
• adjective oncology Harmfully cancerous; as a malignant tumor.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective dangerous to health; characterized by progressive and uncontrolled growth (especially of a tumor)