Biased reproductive success as mediated by humans especially on non-human populations.
Artificial selection differs from natural selection in two basic ways. First, the selective agent in the case of natural selection is the environment generally whereas with artificial selection it is a specific aspect of an organism's environment, that is, humans in the case of domesticated organisms.
The second way it differs is in terms of what phenotype is being selected where with artificial selection it is simply something that for whatever reason pleases the human breeder whereas with natural selection it specifically is adaptations that lead to increased survival and especially increased reproductive success.
Blurring the lines between natural and artificial selection is additional aspects of selection that operate in the course of domestication, which typically results in increased fecundity (reproductive success) at the expense of propensity to survive under not-artificial, i.e., not-domesticated circumstances (assuming that whatever that is being selected for doesn't also interfere with reproductive success).
An additional point worth making is that the consequences of artificial selection are no different from selection for specialization in general where too it often is the case that fecundity is enhanced at the expense of versatility in terms of survival.
Video (Meet the Belgium Blue, cattle bred for "double muscling")
Video (Dogs and their evolution; a bit melodramatic at times, but generally good entertainment)