In astrology the term malefic is used to refer to something that has a detrimental or negative meaning within the context of astrological interpretations.
In practice, the term is usually used in either a strict sense or a general sense:
The Malefics: Mars and Saturn
In the strictest sense, the term “malefic” is usually only used to refer to one of the malefic planets, which were traditionally defined as Mars and Saturn.
This is derived from a convention in ancient Greek texts where these two planets were referred to as “bad-doers” or “evildoers” (kakopoios), because they were viewed as providing negative or destructive significations in an astrological chart.
This is contrasted with the two “benefic” planets, Venus and Jupiter, which were referred to as “good-doers” (agathapoios), because they were viewed as providing positive or constructive significations in a chart.
Within this context, in most traditional texts a reference to a “malefic” or a “malefic planet” is a reference to Mars and/or Saturn.
Generalized Usage of the Term Malefic
In the general sense, the term “malefic” can be used to refer to anything in a chart that is thought to have some negative or potentially bad meaning. So, for example, even though the planet Pluto wasn’t traditionally categorized as a benefic or malefic planet, some astrologers might say that it has some “malefic” significations, by which they would mean that it correlates with negative events or circumstances, such as death for example.
Other bodies or points in the chart may also be labeled as malefic, or capable of providing something malefic, which is functionally just another way of saying “negative” or “detrimental”.
There is also a concept that is used in Indian astrology where some planets can become “functionally malefic” as a result of ruling certain houses, which means that even though they are not inherently one of the malefic planets, they may come to have some negative or malefic influence in the chart based on their condition in it.
Chris Brennan, Benefic and Malefic Planets in Astrology, The Horoscopic Astrology Blog, June 28, 2007.