In astrology, a malefic planet, or simply malefic, is thought to bring bad luck and misfortune to those born within its radius. Mars is considered the lesser malefic, while Saturn is the greater malefic. It is unclear who considers them to be lesser vs. greater malefic or how this impacts the lives of astrologians.
In modern astrology Uranus, Pluto and for some Ceres are also considered malefic when in hard aspect to a personal planet -- that is, either conjuncted in square or in opposition to the personal planet. They are opposed to the benefic planet (Jupiter and Venus) which grant good fortune by contrast.
Easy/soft aspects to these however can be beneficial
References
Malefic planet Wikipedia(Text) CC BY-SA