Source:http://linkedlifedata.com/resource/umls/id/C2985510
NCI: A single nucleotide polymorphism, or SNP (pronounced "snip"), that is used to "tag" a particular haplotype in a region of the genome. As a subset of all of the SNPs in the genome, tagging SNPs can be extremely useful for testing the association of a marker locus with a qualitative or quantitative trait locus in that it may not be necessary to genotype all of the SNPs.