Genetic purging is the increased pressure of natural selection against deleterious alleles prompted by inbreeding.[1]
Purging occurs because deleterious alleles tend to be recessive, which means that they only express all their harmful effects when they are present in the two copies of the individual (i.e., in homozygosis). During inbreeding, as related individuals mate, they produce offspring that are more likely to be homozygous so that deleterious alleles express all their harmful effects more often, making individuals less fit. Those less fit individuals pass fewer copies of their genes to future generations. In other terms, natural selection purges recessive deleterious alleles in inbred individuals.
Purging reduces both the overall number of recessive deleterious alleles and the decline of mean fitness caused by inbreeding (the inbreeding depression for fitness).
The term "purge" is sometimes used for selection against deleterious alleles in a general way. It would avoid ambiguity to use "purifying selection" in that general context, and to reserve "purging" to its more strict meaning defined above.