As applied to the theory of inheritance, the term atomistic describes inheritance in which the entities controlling heredity are relatively distinct, permanent, and capable of independent action; Mendelian inheritance is an atomistic theory because, in it, inheritance is controlled by distinct genes.
The alternative, and historically older, theory is that of blending inheritance. This is the factually erroneous idea that organisms contain a blend of their parent's hereditary factors and pass that blend on to their offspring.
Figure: Mendelian inheritance is atomistic. In (a) there is blending heredity. The genes color blend in their offspring. Mendelian heredity (b): the parental genes are passed on unaltered by the offspring.