Mutational load is the total genetic burden in a population resulting from accumulated deleterious mutations. It is a kind of genetic load.
It can be thought of as a balance between selection against a deleterious gene and its production by mutation. At the equilibrium, a dominant deleterious mutation has a frequency of m/s, where m is the mutation rate and s is the selective disadvantage of the mutation.
In this case the mutational load can be calculated to be equal to the mutation rate. The load expresses the fact that individuals are dying because of the deleterious mutations that arise. The population carries a load of deleterious mutations, which reduce the average fitness of its members.