single nucleotide polymorphisms

Definition of SNP
National Cancer Institute

SNP

(snip)
The most common type of change in DNA (molecules inside cells that carry genetic information). SNPs occur when a single nucleotide (building block of DNA) is replaced with another. These changes may cause disease, and may affect how a person reacts to bacteria, viruses, drugs, and other substances. Also called single nucleotide polymorphism.

Source: NCI Dictionary of Cancer Terms