It is the axial symmetry of the density function of a random variable. A probability distribution is symmetrical when its probability density function is symmetrical with respect to a fixed value x0:

In the unimodal distributions, the asymmetry is right when the values that you move away more from the average are the highest ones (on the right on the horizontal axis), located to the right of the three measures of central tendency (mode, median, mean); other way the asymmetry is said left.
Guseo R., Statistica, CEDAM, 2006.
Leti G., Statistica descrittiva, Il Mulino, 1999.
Editor: Giuliano DI TOMMASO