1) A single nucleotide variation in a genetic sequence that occurs at appreciable frequency in the population.

2) A variant DNA sequence in which the purine or pyrimidine base (as cytosine) of a single nucleotide has been replaced by another such base (as thymine).

3) Single nucleotide polymorphisms (SNPs) are a type of polymorphism involving variation of a single base pair. Scientists are studying how single nucleotide polymorphisms, or SNPs (pronounced "snips"), in the human genome correlate with disease, drug response, and other phenotypes.