What follows is a definition for the word malignant, as people are often confused between the words benign, and malignant.

Malignant:

  1. Harmful, malevolent, injurious
  2. (oncology) Harmfully cancerous, as a malignant tumor

In other words, malignant means bad and not good like benign does. If you have a tumor, you want it to be benign and not malignant.

Source: Wiktionary contributors, "malignant - Wiktionary",http://en.wiktionary.org/wiki/malignant  (accessed January 31, 2011).

Often, the word malignant is used in speaking of harmful health problems. For instance, if a tumor is malignant, it is dangerous. The opposite of malignant in this case would be benign. A benign tumor is one which is not regarded as harmful.