The point at which a body's orbit makes it's closest approach to the Sun.
The point at which a body's orbit is farthest from the Sun is known as Aphelion
Martian perihelion is during the southern hemisphere's summer. Since Mars has a fairly elliptical orbit, this means that the southern summer is short, and the southern winter is longer (than the northern hemisphere). The difference between the insolation between when Mars is closest to the sun, and when it is farthest away is 40%. (Earth is 6% because its orbit is more circular.)