This is a old classification and must be taken somewhat freely. In the end I like Ptolemy's definition of the terms best. According to Ptolemy, a benefic is a planet whose natural influence leads to moderation and balance. A malefic is a planet whose natural influence leads to extremes and imbalance. In this article I mainly refer to dignity and debility by signs.
Planets are dignified when they are in signs in which they rule or are exalted.
They are debilitated when they are in signs opposite these. Any regular beginning text of astrology will list these signs. There are other dignities and debilities as well. See Lee Lehman, Essential Dignities, Whitford Press. And our thanks to Mr. Hand for giving us permission to post his material here.