Random extinction of alleles.
This definition is a bit extremely stated since simply random changes in allele frequencies would also qualify under the heading of impact of genetic drift on genetic variation.
By contrast, note that natural selection too can and does result in extinction of alleles as well as changes in allele frequencies, but unlike genetic drift, that extinction or changes due to natural selection are not random. The impact of natural selection, that is, is deterministic while that of genetic drift is stochastic.
Note too that these ideas can be restated instead as random fixation of alleles, though as stated in terms of extinction, as above, the ideas are more general. That is, allele extinction can occur without necessarily allele fixation since a population does not necessarily possess only two alleles at the locus in question.
In terms of genetic drift, see: impact of population size, impact on allele frequency, impact on genetic variation, and impact on allele fixation.