The point at which a body's orbit is farthest from the Sun.
The point at which a body's orbit passes closest to the Sun is known as Perihelion.
Effects on Mars' Climate
Mars' orbit is more eccentric than Earth's. (Earth is 0.017, where as Mars has 0.093.) This means that when Mars is the furthest from the sun, it gets ~40% less energy than when it is closest. (With Earth the difference is 6%.)