In astrology, a malefic planet, or simply malefic, is thought to bring bad luck and misfortune to those born within its radius. Traditionally, Mars and Saturn are considered the malefic planets, with Mars being the lesser malefic and Saturn the greater. In modern astrology Uranus, Neptune, and Pluto 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 planets such as Jupiter and Venus, which are said to grant good fortune by contrast.
A planet that is placed between two malefic planets, traditionally Mars and Saturn, is said to be "besieged". Besieged planets are believed to be very restricted in their functioning, so even if a planet is a benefic, the fact that it is besieged means the beneficial attributes it would normally emanate will be lessened whilst in that configuration.